The impact of in utero exposure to cancer treatments on foetal reproductive development and future fertility: a systematic review.

Cancer is diagnosed in 1–2 out of every 1000 pregnancies, estimated to lead to the treatment of around 5000 pregnant cancer patients in Europe every year ( Van Calsteren and Amant, 2014 ; de Haan et al. , 2018 ). Since oncological and obstetrical data are not routinely linked, the actual incidence may be underestimated ( Maggen et al. , 2020 ), but the age of first-time parents in Western societies has been steadily increasing since the 1950s, so it is predicted that the rate of cancer diagnoses in pregnancy will continue to rise accordingly. The most common malignancies diagnosed during pregnancy are breast, cervical, and lymphoma ( de Haan et al. , 2018 ), with breast cancer accounting for 40% of cancers diagnosed during pregnancy ( de Haan et al. , 2018 ; Maggen et al. , 2020 ). Breast cancer in pregnant women is often diagnosed at a more advanced stage, possibly due to the pregnancy-associated changes in breast weight, firmness, and density, all of which may impact the initial interpretation of the tumour as well as complicate the clinical examinations and mammogram results ( Durrani et al. , 2018 ). The type of cancer treatment administered to a pregnant cancer patient depends on the type and stage of cancer but can involve chemotherapy, hormone therapy, immunotherapy, radiotherapy, or surgery, or a combination of these ( Cardonick and Iacobucci, 2004 ). For those who are pregnant when receiving a cancer diagnosis, though, treatment can often not wait until after birth ( Maggen et al. , 2019 ). The decision whether or not to treat a pregnant cancer patient requires careful consideration, where the benefit of cancer treatment must be weighed against the potential risk to the foetus, bearing in mind that studies have shown that many chemotherapy drugs have the ability to pass through the placenta and enter the foetal circulation ( Van Calsteren et al. , 2011 ; Dekrem et al. , 2013 ; Köhler et al. , 2015 ; Benoit et al. , 2021 ). Approximately 28% of all cancer patients (including non-pregnant patients) end up receiving chemotherapy treatment ( Cancer Research UK, 2014 ), although this will vary depending on the cancer type, treatment, and patient’s age. For example, in people diagnosed with breast cancer, the proportion who subsequently receive chemotherapy treatment is estimated to be considerably higher, around 70% ( Morimoto et al. , 2010 ). Cancer treatment regimens usually involve the administration of different chemotherapy agents in combination, each drug acting through contrasting mechanisms ( Blommaert et al. , 2020 ). Amalgamating different chemotherapy drugs during a treatment regimen lowers the chances of the development of drug-resistant cancer cells while also providing different anti-cancer benefits to reduce tumour size and prevent metastatic potential ( Mokhtari et al. , 2017 ). Chemotherapy agents primarily act by targeting proliferating malignant cancer cells in the body, but the non-specific nature of chemotherapy agents and poor selectivity for cancerous tissues mean that treatment will often result in a wide range of side effects, including detrimental effects on fertility. Hormonal therapy treatments are also used to treat hormone-sensitive cancers, such as oestrogen receptor (ER)-positive breast cancer ( NHS, 2024 ). Although an international consensus panel in 2010 advised against the use of hormonal therapies for cancer treatment during pregnancy ( Amant et al. , 2010 ; Silverstein et al. , 2020 ), there have been numerous cases of pregnant women receiving hormonal-based anti-cancer therapies prior to this or inadvertently falling pregnant during hormonal treatment ( Barthelmes and Gateley, 2004 ; Buonomo et al. , 2020 ). Until relatively recently, chemotherapy was not frequently used in the treatment of pregnant cancer patients due to concerns of adverse effects to the foetus ( Doll et al. , 1990 ; Cardonick and Iacobucci, 2004 ). Indeed, administration of chemotherapy agents is contraindicated in the first trimester of pregnancy due to increased risk of birth malformations and miscarriage ( Cardonick and Iacobucci, 2004 ). However, delaying treatment can also worsen the prognosis. Reassuringly, when treatment is administered during the second and third trimesters of pregnancy, clinical cohort studies investigating the effects of in utero chemotherapy exposure have not found any association with severe congenital, neurological, or cardiac outcomes in the children born ( Amant et al. , 2012 ; Esposito et al. , 2016 ; de Haan et al. , 2018 ; Blommaert et al. , 2020 ; Wolters et al. , 2021 ), nor is there evidence of increased incidences of adverse pregnancy outcomes, such as late miscarriage, for those undergoing treatment ( Maggen et al. , 2020 ). However, there is a higher incidence of complications such as preterm birth and foetal growth restriction reported ( Maggen et al. , 2021 ), both of which are associated with delayed neurological development and comorbidities later in life ( Blommaert et al. , 2020 ). Overall, most anticancer drugs are now thought to be safe to administer after the first trimester of pregnancy, which has led to an increase in the number of cancer patients who have received cancer treatment whilst pregnant ( de Haan et al. , 2018 ). Despite this research, there is a distinct lack of longer-term studies that have followed up on the potential detrimental impacts of in utero cancer treatments on additional health outcomes of children, in part due to the difficulty of studying these effects within the human foetus and children ( Murthy et al. , 2014 ; Korakiti et al. , 2020 ; Greiber et al. , 2022 ). Even in studies that do investigate longer-term outcomes, little, if any, information is provided on the potential reproductive impact of in utero chemotherapy exposure. There may be many reasons for the paucity of long-term follow-up, including a lack of funding or of awareness of potential fertility implications: in utero -exposed individuals may not wish to participate in such studies, or perhaps the majority of the individuals are too young for their reproductive health to be definitively assessed ( Murthy et al. , 2014 ). Regardless, the lack of data on this matter is concerning given the well-documented damaging effects of chemotherapy treatment on gonadal function in children and adults ( Anderson et al. , 2015 ; Picton et al. , 2015 ; Allen et al. , 2018 ; Jayasinghe et al. , 2018 ; Van Dorp et al. , 2018 ; Spears et al. , 2019 ), and with long-term follow-up studies showing a significant reduction in the number of offspring born to cancer survivors ( Chow et al. , 2016 ). The pre-natal period of mammalian gonadal development is sensitive, complex, and crucial ( Fig. 1 ). It initiates during foetal life with the formation of primordial germ cells (PGCs), which invade the developing gonadal ridge and proliferate rapidly. In the ovary, PGCs initiate meiosis, then form primordial follicles (PMFs) ( Hartshorne et al. , 2009 ). PMFs become abundant in the human ovary from around 16–20 weeks of gestation (embryonic Day 16.5 [E16.5] in the mouse), and all PMFs will have formed before the end of the third trimester. The number of PMFs present at birth represents the pool from which all future ovulated follicles will come, i.e. the ovarian reserve. The PMF pool is therefore directly related to the future fertility and reproductive lifespan of that individual. In the developing foetal testis, Sertoli cells appear from 7 weeks of gestation (E10.5 in the mouse) and act as the central drivers of testis differentiation ( Svingen and Koopman, 2013 ). They differentiate and engulf the PGCs to form seminiferous cords ( Allen et al. , 2018 ). The germ cells (gonocytes) then differentiate into pre-spermatogonia, while the Sertoli cells, the somatic cells of the testis that are essential for supporting spermatogenesis in later life, actively proliferate during foetal life ( Allen et al. , 2018 ). Their rate of proliferation at this stage and again at puberty will influence the size of the adult population of Sertoli cells and the quantity of sperm produced. The Sertoli cells also drive the differentiation of the foetal Leydig cells, which produce testosterone, and later on, also drive the development of peritubular myoid cells. Testosterone production is key for the masculinization process of the foetal testes and, in particular, during the critical period of masculinization programming window (MPW) around 8–14 weeks gestation. Impaired testosterone production or insult to key cells during this sensitive period can have a detrimental impact on male reproductive health ( Sharpe, 2020 ). Developmental stages of ovarian and testicular development in the mouse and human. Yellow shaded area: the time period considered safe to administer chemotherapy during pregnancy (during the second trimester). PGCs, primordial germ cells; DPC, days post-coitum; GW, gestational week; PND, postnatal day. Adapted from figures and information in Svingen and Koopman (2013) , Hartshorne et al. (2009) , Allen et al. (2018) , and Ozcan et al. (2023) . Any compound with the ability to interfere with these vulnerable early stages of gonadal development, such as chemotherapeutic or hormonal therapies, has the potential to induce detrimental effects on the subsequent fertility of an in utero -exposed individual. Given the well-known gonadotoxicity of many chemotherapy drugs ( Allen et al. , 2018 ; Spears et al. , 2019 ), there is a concerning possibility that children born to mothers who had chemotherapy treatment during pregnancy might have fertility complications later in life. However, only a handful of studies to date have explored the effect of chemotherapy agents on the foetal gonads. Therefore, this review aimed to systematically search for all studies reporting on the reproductive outcomes following chemotherapy or hormone therapy exposure to developing gonads. The work reviews the current evidence base for the possible impact of anti-cancer drugs on the developing gonads and/or on reproductive outcomes in both human and animal model studies to understand the impact of in utero chemotherapy or hormone therapy exposure on foetal reproductive development.

The protocol for this systematic review was registered a priori with the PROSPERO register (CRD42021272882 and CRD42021271892) and followed the PRISMA 2020 guidelines for systematic reviews ( Page et al. , 2021 ). Since this review sought to evaluate evidence from both animal model and human-based studies, two separate text searches were conducted, each with appropriate inclusion and exclusion criteria. The full search strategy is presented in Supplementary File S1 . Both in vivo and in vitro study designs for human and animal model-based research were included. For inclusion in the review, a study had to contain outcomes relating to the reproductive health or fertility of the exposed groups; studies were excluded if an appropriate control was not used. Human-based studies were included if foetal tissue or individuals were exposed to any chemotherapy drug in utero or during the prenatal period (and had not been exposed to any other cancer treatment such as radiotherapy). Studies were excluded if there had been any exposure of the child to chemotherapy after birth. For in vitro experiments investigating pro-drugs, papers were only included where the active metabolite(s) were added to the medium. Animal-model studies in rodents were included if the exposure ( in vivo or in vitro ) was up to and including postnatal Day 4 (PND4), which has been shown in rodents to be the equivalent to human foetal gonadal development towards the end of the third trimester ( Hartshorne et al. , 2009 ; Stefansdottir et al. , 2016 ; Allen et al. , 2018 ). All included studies were conducted on mammalian tissues, specifically human, mouse, or rat. A keyword search was used to return animal and human studies from the databases PubMed, Web of Science, and Google Scholar, with publication dates up to July 2024 ( Supplementary File S1 ). The returned articles from each database were then imported into Covidence reference manager software ( Covidence, 2024 ), where duplicates were removed, and the final number for title and abstract screening was presented. Screening, using the inclusion and exclusion criteria, was carried out separately for the animal model and human studies. Two independent reviewers undertook the first title and abstract and then full-text screening stages. Any discrepancies were discussed between reviewers to decide on final inclusion or exclusion. The final included studies from both the animal model and human searches underwent independent data extraction by two reviewers and were then merged into a single spreadsheet collated from the agreed data. The data extracted for both animal model and human studies included study characteristics: year of publication, study design, methods, aim, primary outcomes, and key findings. The specific drug exposure and reproductive outcomes of interest were then extracted. This included sample or population information, intervention and chemotherapy used, timing of administration, dosage, and the findings of reproductive outcomes measured in the study. Data on in utero hormonal therapy use and reproductive outcomes were also extracted, as this was used as treatment for pregnant women in some cases before it was advised against in 2010 ( Silverstein et al. , 2020 ). The key outcomes of each article were summarized in a main table of reproductive outcomes and tabulated by chemotherapy or hormonal drug and class. Heterogeneity within the reporting of data and limited comparative values meant that meta-analyses of the data were not possible. Given that, narrative synthesis of the trends seen and discussion of future recommendations in light of the results presented was undertaken. Results from both human and animal model studies were grouped together for interpretation. Direction of effect (significant differences) and change were presented for each outcome extracted and merged for those studies reporting the same effect. The studies investigating hormonal therapies and their outcomes were collated in a separate table. Data from each study were grouped by drug and analysed narratively to examine the evidence presented for adverse reproductive outcomes following cancer treatment exposure in utero. All included studies were assessed by the independent researchers for possible sources of bias and for quality of results. Comments on the validity and justification of conclusions, as well as the methods used, were recorded in the characteristics table. Each included study was also assessed for quality and bias using the validated tool, SciRAP tool for in vivo toxicology studies—assessment sheet or SciRAP tool for in vitro toxicology studies—assessment sheet, where appropriate, versions 2.3 ( Karolinska Institute, 2024 ). The final score for each study was included in the study characteristics table ( Supplementary Table S1 ). One study was removed following full-text inclusion due to poor-quality methods and results ( Saxena and Singh, 1999 ). The SciRAP tool does not include a specific tool to categorize the reliability of the studies from their scores; therefore, we established a method of converting these scores into reliability groups, as found in other studies using this tool ( Wiklund and Beronius, 2022 ).

The stages of study selection and screening are presented in the PRISMA diagrams ( Fig. 2 ). Following full-text review, one study that initially met our study criteria was excluded ( Yu et al. , 2014 ), as it demonstrated fundamental discrepancies between described tissues and the figures included, which discredited their results to the reviewers. PRISMA diagram. Results of search strategy and screening in this systematic review, detailing both ( a ) human and ( b ) animal studies. Both searches looked for studies of chemotherapy and hormonal therapy exposures. Created using Covidence reference software ( Covidence, 2024 ). Four human-based studies met the inclusion criteria, along with 22 animal model-based studies (total n = 26). The full data extraction table with characteristics of included studies and outcomes of interest can be found in Supplementary Table S1 . Based on their SciRAP methodological scores ( Supplementary Table S2 ), 24 of the included studies were classed as reliable, or reliable with restrictions. Two of the included studies were classed as having low reliability. The studies included in this review investigated exposure to a range of clinically relevant chemotherapy agents and hormone-based therapies that may be used for the treatment of pregnant cancer patients, outlined in Table 1 . The cancer drugs used in included studies and the evidence for their placental transfer rate. Some studies may have used more than one drug in their exposures. Total number of studies = 26, evidence from placental transfer data taken from Köhler et al. (2015) , Benoit et al. (2021) , and Triarico et al. (2022) . FTR, foetal transfer rate. A total of 13 studies examined the impact of chemotherapy agents on foetal gonadal and/or reproductive health, together reporting on 23 different reproductive outcomes that included analyses of gonadal cell type number and health, hormone levels, anogenital distance (a readout of foetal testosterone production), and levels of gonadal apoptosis and oxidative stress (summarized in Table 2 ). Reproductive outcomes and direction of effect following in utero exposure to chemotherapy drugs. Up/down arrows denote significant differences in outcome to control groups ( P <0.05). The horizontal bidirectional arrow denotes no significant difference. E, embryonic day; F2, second filial generation; GW, gestational week; PND, postnatal day; GD, gestational day. Seven of those studies examined foetal testicular effects, and six of the studies examined effects on the foetal ovary. Four of these studies were based on human tissues, whilst the other nine used rodent models (rats or mice). Of these, several studies demonstrated a significant decrease in germ cell number following chemotherapy exposure, in both human and rodent studies ( Morgan et al. , 2013 ; Comish et al. , 2014 ; Lopes et al. , 2014 ; Stefansdottir et al. , 2016 ; Tharmalingam et al. , 2020 ; Matilionyte et al. , 2022a , 2023 ). This significant decrease was reported in the human foetal testis following in vitro exposure to both cisplatin and carboplatin ( Tharmalingam et al. , 2020 ; Matilionyte et al. , 2022a , 2023 ), as well as the mouse ovary after in vivo exposure to cyclophosphamide ( Comish et al. , 2014 ) and in vitro exposure to etoposide, doxorubicin, docetaxel, and cisplatin ( Morgan et al. , 2013 ; Lopes et al. , 2014 ; Stefansdottir et al. , 2016 ; Ozcan et al. , 2023 ). One study found a detrimental impact on testicular Leydig cell number and consequently, reduced testosterone levels, following cytarabine exposure ( Namoju and Chilaka, 2022 ). The Sertoli cell population was unchanged in cultured second-trimester human testis tissue treated with cisplatin ( Matilionyte et al. , 2022b ), or after in vivo busulfan exposure to the foetal rat testis ( Hall and Gomes, 1973 ). Increased levels of apoptosis were observed in second-trimester human foetal testicular gonadal tissue following cisplatin exposure ( Matilionyte et al. 2022b ). Mouse and rat testis weights were also significantly reduced, and testicular architecture was disrupted by the chemotherapy agents cyclophosphamide, cytarabine, and busulfan in vivo ( Hall and Gomes, 1973 ; Comish et al. , 2014 ; Namoju and Chilaka, 2022 ). In the ovary, in utero cyclophosphamide increased follicular activation once mice had reached adulthood ( Comish et al. , 2014 ). In another in vitro exposure study of the foetal mouse ovary, etoposide caused meiotic disruption and germ cell loss, which subsequently impacted the number of follicles formed ( Stefansdottir et al. , 2016 ). Furthermore, doxorubicin exposure at PND2 caused a 91% and 95% loss of oocyte density compared to controls at PND4 and PND7, respectively ( Ozcan et al. , 2023 ). On the other hand, this study found that exposure of mouse PND2 ovaries to cisplatin, docetaxel, and paclitaxel did not lead to significant oocyte loss compared to controls. Overall, the combined results from the included studies provide evidence that in utero exposure to different types of chemotherapy drugs can have a detrimental impact on many key gonadal cell populations and reproductive processes in both males and females ( Table 2 ). However, there were also instances where chemotherapy exposure did not cause significant aberrations when compared to controls. Docetaxel and paclitaxel were not found to induce significant germ cell loss in the mouse ovary at PND14 and 30 following in vivo exposure on embryonic Day 16.5, although an increase in the number of atretic and apoptotic follicles was found ( Chaqour et al. , 2024 ). A single exposure to busulphan at gestation Day 13 did not significantly change the number of Leydig cells or Sertoli cells in the rat testis ( Hall and Gomes, 1973 ). Sertoli cell number was also not affected following in vitro cisplatin exposure of the human foetal testis ( Matilionyte et al. , 2022b ). Encouragingly, there were several instances of consistent findings between human and rodent model studies, supporting the applicability of animal studies to model human studies in this case. Hormonal cancer therapies are used in the treatment of particular cancers, for example, hormone receptor-positive breast cancers, which have either oestrogen or progesterone receptors. For these cancers, drugs such as tamoxifen are used to inhibit the ER activity ( Kazimir et al. , 2023 ). Consequently, this review also examined studies that investigated the effects of in utero exposure to hormonal cancer therapies ( Table 3 ). Reproductive outcomes and direction of effect following in utero exposure to hormonal cancer therapies. Up/down arrows denote significant differences compared to control groups ( P <0.05). The horizontal bidirectional arrow denotes no significant difference. E, embryonic day; GD, gestational day; dpp, days post-partum; FSH, follicle-stimulating hormone; PND, postnatal day. A total of 13 studies examined the foetal reproductive impact of hormonal drug exposure. All studies used rodent models with exposure to either tamoxifen or letrozole. These studies and effects reported are summarized in Table 3 . Tamoxifen exposure significantly reduced the number of germ cells in the testis after a single dose ( Gonzalez-Gonzalez et al. , 2017 ), with similar effects found in the ovary following both single and repeated tamoxifen exposures ( Razvi et al. , 2007 ; Roshangar et al. , 2010 ; Zhao et al. , 2023 ). Likewise, letrozole exposure also reduced germ cell number in rat foetal testes, resulting in a significant reduction in spermatogenesis in adulthood ( Shaaban et al. , 2023 ). In addition, testosterone levels in rat male offspring were reported to have significantly increased at PND60, after three doses at varying concentrations given between gestational Days 16 and 18 in vivo , resulting in an increased anogenital distance ( Shaaban et al. , 2023 ). Studies examining other cell types within the testis found a reduction in Sertoli cell number following a single tamoxifen dose at birth, but no impact on Leydig cell number ( Gonzalez-Gonzalez et al. , 2017 ). Both tamoxifen and letrozole caused disruption of the testicular architecture in rats ( Gonzalez-Gonzalez et al. , 2017 ; Shaaban et al. , 2023 ). Detrimental effects were also found in hormone therapy-exposed foetal ovaries, where a significant increase in apoptosis, DNA double-strand breaks, and epigenetic modifications were reported in mouse ovaries after a single in utero tamoxifen dose ( Zhao et al. , 2023 ). A significant decrease in PMF numbers and corpora lutea was observed following foetal exposure to tamoxifen in vivo ( Razvi et al. , 2007 ; Zhao et al. , 2023 ). Interestingly, the number of germ cell nests was significantly increased in the mouse ovary after foetal tamoxifen exposure ( Roshangar et al. , 2010 ). Uterine and ovarian weights were also significantly reduced after tamoxifen exposure in mice ( Hilakivi-Clarke et al. , 2000 ; Karlsson, 2006 ; Razvi et al. , 2007 ; Parandin et al. , 2016 ).

The evidence presented here from both in vitro and in vivo studies of exposure to chemotherapy and hormone therapy in developing gonads and other reproductive tissues, in both rodent models and human foetal tissue, points towards the possibility of adverse reproductive development and fertility as a result of exposure. The most consistent effect observed in the gonads of both sexes, across several studies, models, and drug types, was the detrimental impact found on germ cell number. Exposure to tamoxifen reduced both ovarian and uterine weight whilst increasing the number of germ cell nests within the ovaries and reducing the number of PMFs formed. The results of this systematic review also emphasize the distinct lack of published clinical research investigating the impact of anticancer agents on foetal reproductive development in utero , both in the short and long term. Taken together, our data demonstrate the real probability that the foetal gonad could be vulnerable to cytotoxic insult during maternal chemotherapy and/or hormone treatment, potentially affecting future fertility. Ultimately, improved understanding in this area will offer clinicians and pregnant cancer patients accurate and up-to-date information about the impact of chemotherapy treatment on the foetal reproductive system to inform decisions and mitigate risk to the foetus and mother. Moreover, we emphasize the importance of following up and assessing key reproductive indicators in children and young adults who have been exposed to cancer treatments in utero .

To the best of our knowledge, this is the first systematic review evaluating the evidence for the impact of in utero exposure to cancer treatments on foetal reproductive development and the potential implications for subsequent reproductive function and fertility. To date, studies investigating the adverse effects of in utero chemotherapy exposure on the offspring have primarily focused on postnatal outcomes such as neurological and cardiac effects or on major congenital malformations ( Murthy et al. , 2014 ; de Haan et al. , 2018 ; Safi et al. , 2019 ; Blommaert et al. , 2020 ), but have not reported on reproductive outcomes of the individuals who were exposed to cancer treatments in utero. The studies included in our systematic review demonstrate evidence of the significant vulnerability of foetal reproductive development to chemotherapy and hormone therapy exposure. One of the key considerations for any study examining the impact of in utero exposure to cancer treatments is understanding their rate of placental transfer and the level of exposure for the foetus. Available experimental placental perfusion studies have demonstrated that chemotherapy drugs are able to pass through the placenta and enter the foetal blood stream ( Benoit et al. , 2021 ; Triarico et al. , 2022 ). One of the chemotherapy drugs examined in several of the studies in this review, cisplatin, has a low molecular weight but a high capacity to bind to plasma proteins ( Van Calsteren et al. , 2010 ). Whilst few studies have explored the transplacental transfer of cisplatin, one study using a human placenta found cisplatin concentrations as high as 42% and 65% of that of the maternal blood in the amniotic fluid and umbilical artery, respectively ( Köhler et al. , 2015 ), suggesting that cisplatin may be one of the chemotherapy agents with the highest rate of transplacental transfer. In comparison, cyclophosphamide showed a 25% foetal transfer rate in a baboon model of placental transfer, and 25% of maternal plasma drug levels were detected in the amniotic fluid of an in vivo human pregnancy ( Van Calsteren and Amant, 2011 ). Paclitaxel showed no evidence of placental transfer and very low (1.5%) transfer in mouse and baboon placental models, respectively, but was detected in neonates’ meconium ( Benoit et al. , 2021 ). The known placental transfer rates and detections in the foetal compartment are illustrated in Fig. 3 , demonstrating the variation in potential exposure of the foetus to chemotherapy administered to the pregnant mother. Schematic to demonstrate the variation in placental perfusion of different chemotherapy drugs. Data from mouse, baboon, and human models of commonly used chemotherapies in pregnant cancer patients that pass through the placenta. Percentage in foetal compartment as compared to maternal plasma concentration ( Benoit et al. , 2021 ; Van Calsteren and Amant, 2011 ). The data gathered from the studies included in this review demonstrate the foetal gonadotoxicity of a number of chemotherapies and hormone therapies. The most consistent effect observed in the gonads of both sexes, across several studies, models, and drug types, was the detrimental impact found on germ cell number. Furthermore, in females, a reduction in the number of PMFs formed within the ovary, and in some cases, premature activation of these follicles, was observed in some of the studies. This could be associated with a diminished follicle pool, early menopause, or premature ovarian insufficiency (POI). Indeed, mice prenatally exposed to the chemotherapy drugs docetaxel and paclitaxel had fewer litters overall, with the mice in the paclitaxel group also having their last litter earlier ( Chaqour et al. , 2024 ), potentially reflecting POI. Moreover, loss of ovarian activity might not only result in sub- or infertility but also persistent oestrogen deficiency can affect bone function, cardiovascular, and neurological health ( Spears et al. , 2019 ). Exposure to tamoxifen reduced both ovarian and uterine weight, whilst increasing the number of germ cell nests within the ovaries and reducing the number of PMFs formed. These results are consistent with the oestrogenic activity of tamoxifen, which binds to ERs and produces both oestrogenic and anti-oestrogenic effects ( Farrar and Jacobs, 2023 ). Interestingly, mice exposed to the chemotherapy agent docetaxel in utero were also found to have an increased number of multi-oocyte follicles in their ovaries ( Chaqour et al. , 2024 ). Indeed, numerous studies have demonstrated that exposure of the developing foetal ovary to oestrogenic compounds disrupts cyst breakdown, giving rise to multiple oocyte follicles whilst reducing the number of PMFs formed and reducing the size of the follicle pool ( Jefferson et al. , 2002 , 2006 ; Chen et al. , 2007 , 2009 ; Pepling, 2012 ). Chemotherapy drugs and hormone therapy agents resulted in reduced testicular weight in all studies reporting on this outcome, apart from one. Furthermore, the studies examining the effects of chemotherapy exposure on human foetal testicular tissue suggest that the timing of exposure within the second trimester could be of particular importance, where germ cell numbers in the early second trimester testis were not impacted by treatment, but with a detrimental effect found in testicular tissue from later stages of the second trimester ( Matilionyte et al. , 2022a ). This suggests that the human foetal testis might be more vulnerable to chemotherapy exposure in the later stages of the second trimester. This could be due to differences in the rate of proliferation in testicular cell (sub)types, rendering them more vulnerable at certain time points. During the second and third trimesters, there is a gradual transition of the germ cell population from gonocytes to (pre)spermatogonia. The changes in germ cell sensitivity to cisplatin during the second trimester may be due to the variation in the proliferation rate between these germ cell sub-populations and the changes in proliferation rate in individual germ cell populations over time ( Mitchell et al. , 2010 ; Matilionyte et al. , 2022a ). Given that many of the key gonadal cell types are undergoing consistent and rapid proliferation in foetal life, it is possible that cancer treatments that are designed to target rapidly dividing cells could have a detrimental effect on the proliferation of these cells during gonadal development. Indeed, the germ cells in both the ovary and testis go through rapid phases of proliferation, which render them more vulnerable to the types of chemotherapy drugs that specifically target rapidly proliferating cells ( Tharmalingam et al. , 2020 ; Matilionyte et al. , 2022a , 2023 ). However, the gonadotoxicity of the drugs was not only confined to the germ cells, where both Leydig- and Sertoli cells of the foetal testis were shown to be detrimentally affected by chemotherapy agents and hormone therapy treatments, respectively. Nevertheless, loss of either the spermatogonial stem cells or the supporting somatic cells can have a detrimental impact on long-term fertility. The hypothalamic–gonadal–pituitary axis is established during foetal life, and consequent disruption to this, via chemotherapy or hormonal therapies, could lead to detrimental effects on fertility in later life. There appears to be a consistent disruption to the cellular functions of the testis, causing changes to the organ weight and hormone production, both of which could have long-lasting impact into adulthood. Evidence from both human and animal studies has shown that disruption to the development of the reproductive system during foetal life can result in the onset of reproductive disorders in later life, such as testicular dysgenesis syndrome (TDS) in males and endometriosis or premature ovarian failure in females ( Rhind et al. , 2001 ; Main et al. , 2006 ; Monniaux, 2018 ). Indeed, exposure to gonadotoxic and/or oestrogenic compounds during the MPW, which occurs between E15.5–18.5 in rats, and estimated to be 8–14 weeks of human gestation, and a subsequent impairment of androgen production in the foetal testis, has been strongly associated with TDS in later life ( Sharpe, 2020 ). The exact mechanism of TDS is unknown, but suppression of testosterone production, androgen receptor expression, or Leydig cell function in the foetal testis has been associated with subsequent reproductive impacts in adulthood, including cryptorchidism, hypospadias, dysfunctional testis, as well as testicular cancer and impacts on fertility ( Skakkebaek et al. , 2001 ; Sharpe, 2003 ). In this systematic review, only two studies were identified that reported the impact of chemotherapy drugs on Leydig cell number and testosterone levels, with both of these drugs, cytarabine and busulfan, showing a detrimental effect on these reproductive outcomes; importantly, the exposure windows in both studies overlapped with the MPW ( Hall and Gomes, 1973 ; Namoju and Chilaka, 2022 ). For hormonal cancer treatments, only one study explored the impact of tamoxifen on Leydig cell number, reporting no detrimental impact ( Gonzalez-Gonzalez et al. , 2017 ). Given the distinct lack of data on the impact of cancer treatments on Leydig cells and/or testosterone production, combined with the lack of long-term follow-up studies on reproductive health, there remains a distinct gap in knowledge about how these cancer treatments might be impacting the reproductive health of males exposed in utero . This is particularly pertinent given the well-known oestrogenic effects of tamoxifen ( Farrar and Jacobs, 2023 ). Whilst the majority of the studies investigated effects on the foetal reproductive systems, it is important to consider the possibility of a transgenerational ‘grand-maternal effect’ if the drug elicits genetic damage in the exposed foetal germ cells that could be passed on to subsequent generations. One study demonstrated multi-generational effects on ovarian health, where the female offspring of female mice that were exposed to docetaxel in utero (E16.5) had increased levels of atretic secondary follicles ( Chaqour et al. , 2024 ). Many chemotherapy drugs, including cisplatin, doxorubicin, etoposide, and cyclophosphamide, act by inducing double-stranded DNA breaks within the cancer cell, activating the apoptotic machinery ( Halim et al. , 2018 ; Wang et al. , 2018 ; Gold and Raja, 2023 ; Yadav et al. , 2023 ). In the studies reviewed here, there was evidence of increased levels of cellular apoptosis (albeit contradicting) and oxidative stress in chemotherapy-treated gonads ( Tharmalingam et al. , 2020 ; Matilionyte et al. , 2022b ; Namoju and Chilaka, 2022 ; Chaqour et al. , 2024 ), as well as increased levels of DNA double-stranded breaks and epigenetic effects following tamoxifen exposure ( Zhao et al. , 2023 ). It is important to note that not all germ cells suffering genetic insult undergo apoptosis, which means that in the ovary, this could result in oocytes progressing through meiosis despite the presence of DNA damage ( Marangos et al. , 2015 ). It is theoretically possible, therefore, that damaged germ cells could go on to form follicles, ovulate, and potentially result, for example, in an aneuploid embryo in the second generation, which would have devastating consequences for both the parents and the offspring. Taken together, there is a distinct possibility that the foetal gonad could be vulnerable to cytotoxic insult during maternal chemotherapy and/or hormone treatment, potentially affecting future fertility. Delayed clinical recognition is very likely and might not present until adolescence, while more subtle effects on fertility would not be evident until much later, potentially when the person wishes to have a child themselves or if they enter early menopause. This systematic search looked for all published documents in a range of databases and provides a comprehensive overview of the evidence at the time for the effects of in utero chemotherapy exposure. However, there is inevitable variability in quality between the different studies included in this review, further complicated by an inability to test the robustness of each of the findings. There will also be inherent publication bias in our findings, with the publication process often favouring studies reporting significant experimental findings over those reporting negative findings. In addition, we have been unable to group and compare the data presented by each study to perform a meta-analysis, due to the heterogeneity of the results, with the studies using differing doses, drugs, and animal models. Findings from the animal model studies cannot be directly extrapolated to humans, due partly to interspecies differences in metabolism and placental anatomy, as well as the fact that rodents are polyovular, delivering litters, while humans are mono-ovular, most often having a singleton pregnancy. Given the limitations in extrapolating findings from rodent models, further research will be needed in other reproductive developmental study models or, ideally, in follow-up studies on the offspring of the pregnant cancer patients themselves. For the purpose of this review, a decision was made to include all studies that covered any gestational stage, including any studies up to and including PND4 in the rodent models. This is because that particular time window has been found to be more closely aligned to the developmental processes and stages of pre-natal human gonadal development ( Hartshorne et al. , 2009 ; Allen et al. , 2020 ). Similarly, the in vitro -based exposure studies will be limited in their application for understanding the actual reproductive impact of chemotherapy, as they do not replicate exposures in utero or longer-term effects to foetal tissue that may be seen following exposure. Moreover, experimental studies often focus on the short-term effects on cells and tissues and do not look at the long-term impacts, such as sperm production, fertility, or potential transgenerational effects. We have been unable to identify any clinical cohort studies analysing reproductive outcomes among those exposed to cancer treatment in utero . The human studies identified perform in vitro exposure of foetal tissues, which limits extrapolation of these results more widely but does give valuable insight into the effects of chemotherapy agents on human foetal gonadal tissues. Moreover, as previously mentioned, chemotherapy and hormone therapy drugs are usually administered in a combination of drug classes and regimens. This is not reflected in any of the studies included in this review, where all the studies investigated single chemotherapy or hormonal therapy exposures. In addition, the doses used in experimental studies may not accurately reflect real-life foetal exposure levels. Consequently, there are currently no available data on the impact that a cocktail of drugs administered to a pregnant cancer patient may have beyond those seen in the included studies. The results of this systematic review emphasize the distinct lack of published research investigating the impact of anticancer agents on foetal reproductive development in utero . The majority of the available data come from animal models, with only 4 out of the 26 studies included in the review having been conducted on human tissues, specifically the human foetal testis. As of yet, no published studies have examined the impact of cancer treatments on human foetal ovarian tissue. The remainder of the studies included in the review were carried out on mouse and rat models, which, whilst providing an important basis for beginning to understand the potential detrimental effects of in utero exposure to cancer therapies on gonadal development, highlight the lack of available data in this area. Hormonal therapy is now contraindicated during pregnancy; therefore, the priority should be to evaluate the impact of chemotherapy agents on the developing foetal gonads and on the future reproductive health of individuals exposed in utero . Work should focus on those drugs considered acceptable to administer to pregnant cancer patients and commonly used in that patient group ( Esposito et al. , 2016 ). The currently available follow-up studies on children who have been exposed to chemotherapy drugs in utero have not examined or reported on reproductive outcomes ( Greiber et al. , 2022 ; Huis In ’t Veld et al. , 2024 ). As a consequence, the lack of available data on reproductive impact could potentially result in undiagnosed sub- or infertility. Counselling on the potential adverse reproductive effects of chemotherapy treatment is often prioritized for adult or child cancer patients, but this is likely not the case for those born after in utero exposure to the same gonadotoxic treatments. Nevertheless, reproductive health is an important aspect of an individual’s long-term health and wellbeing and is the top non-survival-related concern among reproductive-age women who face cancer treatment ( Xie et al. , 2022 ). The long-term reproductive health of in utero -exposed children should therefore be taken into consideration by healthcare providers and researchers. A diagnosis of cancer during pregnancy may be further complicated in certain countries where abortion is not legal, including in parts of the USA, where recent changes in abortion laws following the overturning of Roe. v. Wade has led to increased ethical and legal concerns surrounding the treatment of pregnant women with cancer ( Arup and Shravan, 2023 ). In these places, restricted access to abortion due to medical reasons, such as cancer treatment, where the patient will urgently require treatment, will further complicate an already challenging diagnosis and treatment plan, potentially leading to a further rise in the number of children being exposed to cancer treatments in utero. By carrying out this systematic review, we have demonstrated the distinct lack of clinical studies that have explored this effect, yet all the relevant studies included in this review point towards a detrimental impact on reproductive development in both sexes following in utero exposure to cancer treatments. With this in mind, it is clear that further research on appropriate animal models and human tissues must be conducted in order to better understand the impact on future fertility. Moreover, current long-term studies investigating the outcomes of individuals exposed in utero ( Greiber et al. , 2022 ; Huis In ’t Veld et al. , 2024 ) should place fertility among the key indicators of longer-term health and wellbeing measures in their in utero -exposed cohort. Without these data, we remain unaware of any potential cohort findings on reproductive health and fertility outcomes. Understanding this impact would enable more accurate information to be given to those deciding on cancer treatments during pregnancy and would also enable those who have already been exposed to chemotherapy in utero to access timely fertility care and potential preservation treatments if this is required, such as cryopreservation of oocytes, testicular tissue, or sperm ( Chen et al. , 2023 ).