Obstetric and Perinatal Outcomes in 44,118 Singleton Pregnancies: Endometrial Preparation Methods for Frozen-Thawed Embryo Transfer.

The first successful live birth, which was a consequence of frozen embryo transfer (FET), was reported in 1984. 1 By separating the oocyte retrieval cycle from the embryo transfer (ET) cycle, FET mitigates the risk of ovarian hyperstimulation and improves synchronization between the developmental stage of the embryo and endometrium, thereby enhancing implantation rates. 2 3 4 The frequency of FET cycles has rapidly increased worldwide, owing to these advantages of FET, advancements in laboratory techniques such as vitrification, implementation of the freeze-all strategy, and utilization of preimplantation genetic testing. 5 6 7 8 In the USA, FET accounts for approximately 30% of all assisted reproductive technology (ART) cycles, while in Europe, it constitutes roughly 40% of all ART cycles. 9 10 FET can be categorized into three types based on the method of preparation of the endometrium: artificial cycle FET (AC-FET), natural cycle FET (NC-FET), and stimulated cycle FET (SC-FET). AC-FET involves creating an endometrial environment for implantation using exogenous estrogen and progesterone, followed by ET. NC-FET entails ET without the administration of any drugs to induce ovulation, relying instead on the confirmation of a surge in endogenous luteinizing hormone in patients with natural ovulation. SC-FET is defined as ET performed after inducing follicular growth using clomiphene, aromatase inhibitors, and gonadotropins. In 2021, a Cochrane collaboration conducted a meta-analysis to evaluate the effectiveness and safety of FET compared to fresh ET, in light of the increased utilization of FET over the past decade. 7 The findings revealed comparable prevalence rates of gestational diabetes mellitus (GDM), preterm delivery, small-for-gestational-age babies, congenital abnormalities, and perinatal mortality between the two approaches. However, FET was associated with a higher risk of hypertensive disorder (odds ratio [OR], 2.15; 95% confidence interval [CI], 1.42–3.25) and large-for-gestational-age babies (OR, 1.96; 95% CI, 1.51–2.55) compared to fresh ET. Furthermore, a sub-analysis of FET indicated a higher risk of preeclampsia in patients undergoing AC-FET compared to SC-FET. 7 Recently, a few studies have investigated the obstetric and perinatal outcomes after FET using national databases, 10 11 12 13 whose findings are consistent with the previous Cochrane review. However, these studies could not conclusively determine the optimal option for FET, owing to limitations such as a small sample size and low methodological quality. Throughout the world, there has been a gradual increase in the proportion of women conceiving at older ages, leading to a rise in ART procedures. In particular, the utilization of FET has been increasing. While studies have investigated the obstetric and perinatal outcomes related to infertility treatments, large-scale studies are still lacking, particularly within the Republic of Korea. Therefore, this study aimed to investigate the obstetric and perinatal outcomes in singleton deliveries following FET cycles based on different approaches of endometrial preparation using a national cohort database from the Republic of Korea.

The study analyzed data extracted from the National Health Insurance Service (NHIS), which operates under the Ministry for Health, Welfare, and Family Affairs in the Republic of Korea. The NHIS serves as the primary medical insurer for the majority of the Korean population, covering approximately 97% of Koreans. 14 To guarantee reimbursement, health care utilization data from all medical institutions are submitted and stored in the NHIS database. Established in January 2002, this comprehensive database encompasses information from every hospital, comprising both inpatient and outpatient records categorized according to the International Statistical Classification of Diseases and Related Health Problems, Tenth Revision (ICD-10), as well as claims for healthcare services and pharmacy dispensing according to the codes designated by the Health Insurance Review and Assessment Service of the Republic of Korea. A research database within the NHIS was accessed (NHIS-2023-1616) for this study. This database is made available to medical researchers for the purposes of policy development and academic research. In the Republic of Korea, infertility treatments have been eligible for health insurance reimbursement since October 2017. Consequently, the dataset utilized in this study encompassed all the NHIS claims records between October 2017 and December 2021. The flow chart depicting the study participants is presented in Fig. 1 . This study focused on women diagnosed with singleton pregnancies and obtained their data from the NHISS. The diagnostic codes for singleton pregnancies included ICD-10 codes Z32-35. A total of 1,767,083 individuals were identified using these codes. The exclusion criteria were as follows: cases without diagnostic codes or procedural codes indicating childbirth (Z37, R3131, R3136, R3141, R3146, R4351, R4356, R4361-4362, R4380, R4507-4510, R4514, R4517, R4518), abortion (O00-06) within 360 days from the initial pregnancy diagnosis, or cases with a procedural code (R6560) indicating artificial insemination within 90 days of the initial pregnancy diagnosis. Distinctions between in vitro fertilization (IVF) pregnancies and naturally conceived pregnancies were made based on the ET procedural codes (R6530-6533, R6540, R6550) within 90 days of the initial pregnancy diagnosis. Furthermore, FETs and fresh ETs were distinguished based on the presence of embryo thawing procedural codes (R6502). FETs were categorized into specific study groups based on the prescription history: 1) the AC-FET group, which was prescribed estradiol (active ingredient estradiol valerate) within 30 days before ET; 2) the NC-FET group, which was not given any prescriptions within 30 days preceding ET; and 3) the SC-FET group, which was prescribed any of the following: human chorionic gonadotropin, clomiphene, letrozole, follicle-stimulating hormone, or human menopausal gonadotropin within 30 days preceding ET. IVF/ICSI = in vitro fertilization/intracytoplasmic sperm injection, FET = frozen embryo transfer, fresh ET = fresh embryo transfer, AC = artificial cycle, SC = stimulated cycle, NC = natural cycle. In the current study, the prevalence of pregnancy, obstetric, and perinatal outcomes was assessed using the ICD-10 diagnostic codes. Pregnancy outcomes included abortion (O00-06), preterm birth (Z3700, Z3701, Z3790-3791, O6010-6011, O6030-6031), term birth (Z3702, Z3792), post-term birth (Z3703, Z3793), and stillbirth (Z371). Obstetric outcomes comprised threatened abortion (O200), hydramnios (O40), oligohydramnios (O410), hypertensive disorders of pregnancy (HDP) (O11, O13-15), gestational hypertension (O13), preeclampsia (O11, O14-15), GDM (O244, O249), placenta previa (O44), placenta abruption (O45), placenta accreta (O432), placental insufficiency (O438, O439), postpartum hemorrhage (PPH) (O72), cesarean section (O82), emergency cesarean section (O821), preterm labor (O602), and preterm premature rupture of membranes (PPROM) (O42). Perinatal outcomes included fetal anomaly (O35), fetal death in utero (O364), intrauterine growth restriction (IUGR) (O365), macrosomia (O366), and fetal distress (O368, O68). Additionally, past medical history was determined by the ICD-10 diagnostic codes for hypertension (I10-15), diabetes (E10-14), polycystic ovarian syndrome (PCOS) (E282), and endometriosis (N80) within one year from the initial confirmation of pregnancy. The χ 2 test was employed to compare groups based on categorical data, while ORs and 95% CIs were computed through binary logistic regression, adjusted for age. Visualization was executed using a forest plot and a two-sided test was conducted with a significance level set at 0.05. Statistical analyses were conducted using SAS Enterprise Guide version 7.15 (2017, SAS Institute Inc., Cary, NC, USA) and R statistical language (version 4.3.1; R Core Team, 2023), along with supplementary packages (forest plot). Individual identifiers within the Korean NHIS database were anonymized. The study procedures were approved by the official assessment panel of the Korean Government and Institutional Review Board (IRB) of Pusan National University Hospital (IRB 2304-015-126), adhering strictly to their guidelines. Since the data were deidentified, the IRB waived the necessity for obtaining informed consent.

A total of 44,118 singleton pregnant women who underwent IVF resulting in delivery or abortion were identified between October 2017 and December 2021 ( Table 1 ), of whom 18,322 received FET before pregnancy, while 25,796 received fresh ET. The proportion of AC-FET cycles was the highest at 57.8%, followed by NC-FET at 24.2% and SC-FET at 18%. The 35–39 years age group (45.4%) underwent the highest number of IVF procedures, followed by the 30–34 years age group at 33.4%. The number of FET procedures has been steadily increasing each year, reaching 48% of all ET cycles in 2020, with a notable rise in the number of AC-FET cycles ( Fig. 2 ). Data are presented as number of patient (%) or the mean (SD). ET = embryo transfer, FET = frozen embryo transfer, AC = artificial cycle, SC = stimulated cycle, NC = natural cycle, PCOS = polycystic ovarian syndrome. A P value < 0.05 was considered statistically significant. P value a is for FET vs. fresh ET and P value b is for AC vs. SC vs. NC comparisons. For post hoc analysis, c indicates a statistically significant comparison between AC and NC or AC and SC, while d signifies statistical significance in the comparisons between SC and NC. FET = frozen embryo transfer, AC = artificial cycle, SC = stimulated cycle, NC = natural cycle. The risk of abortion (adjusted odds ratio [AOR], 1.103; 95% CI, 1.058–1.150) was higher, while the likelihood of term birth (AOR, 0.904; 95% CI, 0.867–0.942) was lower with FET compared with fresh ET. Furthermore, the risks of threatened abortion (AOR, 1.324; 95% CI, 1.268–1.382), HDP (AOR, 1.231; 95% CI, 1.122–1.352), preeclampsia (AOR, 1.464; 95% CI, 1.267–1.692), and placenta accreta (AOR, 1.743; 95% CI, 1.316–2.310) were higher with FET compared with fresh ET. Conversely, the risks of GDM (AOR, 0.852; 95% CI, 0.818–0.887) and PPROM (AOR, 0.804; 95% CI, 0.760–0.850) were lower in FET than in fresh ET. FET was associated with a significantly lower risk of IUGR (AOR, 0.787; 95% CI, 0.647–0.958) but a significantly higher risk of macrosomia (AOR, 1.347; 95% CI, 1.095–1.657) compared with fresh ET ( Table 2 ). Values are presented as number (%) or odds ratio (95% confidence interval). Adjusted odds ratio were adjusted for age and past medical history. FET = frozen embryo transfer, fresh ET = fresh embryo transfer, HDP = hypertensive disorders of pregnancy, HT = hypertension, PPH = postpartum hemorrhage, GDM = gestational diabetes mellitus, PPROM = preterm premature rupture of membranes, CS = cesarean section, IUGR = intrauterine growth restriction, FDIU = fetal death in uterus. A P value < 0.05 was considered statistically significant ( * P < 0.05, ** P < 0.01, *** P < 0.001). The risks of abortion and term birth were comparable between the AC-FET and NC-FET groups. However, the risk of preterm birth (AOR, 1.608; 95% CI, 1.024–2.524) before 37 weeks was higher in the AC-FET group compared to the NC-FET group. The AC-FET group faced significantly higher risks of threatened abortion (AOR, 1.733; 95% CI, 1.596–1.881), HDP (AOR, 1.63; 95% CI, 1.358–1.957), preeclampsia (AOR, 1.488; 95% CI, 1.130–1.935), placenta previa (AOR, 1.341; 95% CI, 1.129–1.593), placenta accreta (AOR, 2.533; 95% CI, 1.437–4.467), PPH (AOR, 1.707; 95% CI, 1.447–2.013), and GDM (AOR, 1.164; 95% CI, 1.080–1.255), compared with the NC-FET group. The perinatal outcomes did not differ significantly between the two groups ( Table 3 ). Values are presented as OR (95% confidence interval). Adjusted OR were adjusted for age and past medical history. FET = frozen embryo transfer, AC = artificial cycle, SC = stimulated cycle, NC = natural cycle, OR = odds ratio, CI = confidence interval, HDP = hypertensive disorders of pregnancy, HT = hypertension, PPH = postpartum hemorrhage, GDM = gestational diabetes mellitus, PPROM = preterm premature rupture of membranes, CS = cesarean section, IUGR = intrauterine growth restriction, FDIU = fetal death in uterus. A P value < 0.05 was considered statistically significant ( * P < 0.05, ** P < 0.01, *** P < 0.001). When comparing AC-FET to SC-FET, a higher risk of miscarriage and a decreased rate of term birth were observed with AC-FET. Additionally, AC-FET was associated with higher risks of placenta previa, placenta accreta, and PPH (AOR, 1.373; 95% CI, 1.129–1.670; AOR, 2.029; 95% CI, 1.129–3.645; AOR, 1.266; 95% CI, 1.072–1.496, respectively). Although not statistically significant, there was a tendency for higher risks of HDP (AOR, 1.144; 95% CI, 0.955–1.369) and preeclampsia (AOR, 1.106; 95% CI, 0.845–1.448) in the AC-FET group compared to the SC-FET group. The SC-FET group exhibited a lower risk of miscarriage (AOR, 0.823; 95% CI, 0.743–0.911) compared to NC-FET, while the rate of term birth (AOR, 1.246; 95% CI, 1.130–1.374) was higher with SC-FET than that with NC-FET. The SC-FET group showed higher risks of obstetric outcomes such as gestational hypertension (AOR, 1.382; 95% CI, 1.053–1.816) and PPH (AOR, 1.347; 95% CI, 1.093–1.661) compared with the NC-FET group. Additionally, the risk of IUGR (AOR, 1.587; 95% CI, 1.194–2.108) was higher in the SC-FET group than that in the NC-FET group.

This study found that AC-FET was associated with higher risks of HDP, preeclampsia, placenta accreta, and PPH compared to NC-FET, while the risk of macrosomia did not differ significantly between the two groups. Conversely, SC-FET was associated with a lower risk of miscarriage and higher rate of normal delivery beyond 37 weeks compared to NC-FET. Although the risks of gestational hypertension and PPH were elevated, there were no statistically significant differences in the risks of preeclampsia and placenta accreta between SC-FET and NC-FET ( Fig. 3 ). OR = odds ratio, CI = confidence interval, HDP = hypertensive disorders of pregnancy, PPH = postpartum hemorrhage, GDM = gestational diabetes mellitus, PPROM = preterm premature rupture of membranes, IUGR = intrauterine growth restriction, FDIU = fetal death in utero, AC = artificial cycle, FET = frozen embryo transfer, NC = natural cycle, SC = stimulated cycle. Our findings pertaining to the adverse obstetric outcomes following AC-FET compared to NC-FET align with the findings of research conducted in Denmark, Japan, China, Sweden, Germany, and the USA, which also reported a higher risk of HDP, preeclampsia, placenta accreta, and PPH with AC-FET. 11 12 13 15 16 17 These findings were corroborated by recent meta-analyses, which reinforced the heightened risk of adverse obstetric outcomes associated with AC-FET. 2 18 In AC-FET, estrogen substitution leads to the suppression of the dominant follicle, thereby halting ovulation and the formation of the corpus luteum. von Versen-Höynck et al. 19 20 underscored the significance of the absence of the corpus luteum in the elevated risk of HDP, particularly preeclampsia, associated with AC-FET than that with NC-FET. The absence of the corpus luteum results in the depletion of circulating vasoactive factors, including relaxin and vascular endothelial growth factor, a crucial products for facilitating maternal cardiovascular adaptation during early pregnancy, which could potentially exacerbate the risks of abnormal placentation and HDP. 21 22 23 As observed in Zong et al.’s study, 24 women undergoing AC-FET often have anovulation and endocrine disturbances, notable among which are hyperandrogenism, insulin resistance, and dyslipidemia. These results imply that physiological factors may play a role in the pathogenesis of preeclampsia, potentially outweighing the influence solely attributed to endometrial preparation. 25 26 Furthermore, altered levels of sex steroid hormones in women undergoing AC-FET may affect normal placental development, potentially leading to placenta-related complications. 27 28 Insufficient progesterone levels during early pregnancy could further exacerbate the risk of abnormal placentation by hindering the decidualization process and promoting excessive trophoblast invasion, potentially resulting in placenta accreta. 29 While various hypotheses exist to explain the adverse effects of AC-FET on the obstetric and perinatal outcomes, further basic and clinical studies are needed to establish the causal relationship. From a broader perspective, the artificial endometrial environment created by exogenous hormones, rather than the endogenous hormonal balance observed in natural cycles, may contribute to adverse obstetric and perinatal outcomes. Our findings revealed no significant difference in the risk of macrosomia among the FET cycles based on the endometrial preparation method. This finding contradicts previous research suggesting a higher risk of macrosomia with AC-FET. 2 7 18 This discrepancy may be attributed to the lower prevalence of macrosomia in the Republic of Korea, estimated to be one-third to one-fourth of that in the UK and USA. 30 31 32 Moreover, the high accessibility to medical care in the Republic of Korea facilities timely maternal weight gain intervention during pregnancy, potentially affecting the incidence of macrosomia. Furthermore, while macrosomia is primarily associated with risk factors such as diabetes, maternal obesity, and a family history of macrosomia, biological evidence regarding the influence of exogenous hormone therapy in early pregnancy on fetal body weight is not available. 30 Despite the advantages of AC-FET, such as preventing spontaneous ovulation, averting cycle cancellation, and reducing the patient’s hospital visits, the current and previous studies have demonstrated its potential to cause adverse obstetric and perinatal outcomes. As a result, NC-FET is being considered an option for ovulatory women owing to better maternal and fetal safety. However, NC-FET can be challenging for anovulatory women, particularly those with PCOS. Consequently, SC-FET is finding increasing application as an alternative to AC-FET. 33 SC involves the administration of medication for ovulation stimulation to induce formation of the corpus luteum, mimicking the natural process of follicular development and facilitating endogenous estradiol through ovulation induction. 34 In our research, although SC-FET exhibited a slight increase in adverse obstetric outcomes such as gestational hypertension and PPH compared to NC-FET, the rate of miscarriage was lower and the rate of term birth was higher compared to NC-FET. A few studies that directly compared SC-FET and NC-FET reached conclusions similar to our study. 18 35 Additionally, a randomized controlled trial of ovulatory patients indicated a trend towards slightly higher implantation rates, clinical pregnancy rates, and live birth rates in SC-FET cycles compared to NC-FET, albeit without statistical significance. 36 Furthermore, a recent systematic review and meta-analysis targeting anovulatory women showed significantly higher live birth rates, lower miscarriage rates, and a protective effect against obstetric complications such as preterm birth and preeclampsia with SC-FET compared to AC-FET. 34 This suggests that the presence of the corpus luteum is associated with the development of normal placentation. While the underlying mechanism remains unclear and SC-FET may lead to adverse obstetric outcomes compared to NC-FET, it might be speculated that the non-physiologic hormonal imbalance induced by SC-FET could affect normal placentation, potentially increasing the risk of gestational hypertension or PPH. Additionally, since the SC-FET group included those who required medication to induce ovulation, it is likely that anovulatory women, particularly those with PCOS, were more prevalent in this group compared to the NC-FET group. PCOS itself is known to increase the risk of GDM, HDP, and preeclampsia, 37 , 38 which might contribute to the observed higher risk of gestational hypertension in the SC-FET group. However, as the overall risk associated with SC-FET was not higher than that of AC-FET and considering its lower miscarriage rate and higher live birth rate compared to NC-FET, SC-FET may be considered a favorable option for anovulatory women. The principal strength of our study lies in the utilization of data from a national cohort, enabling large-scale analysis, making it the most recent study after Saito et al.’s study conducted in 2019 (to the best of our knowledge). 11 Additionally, in the Republic of Korea, where a significant portion of the population is enrolled in national health insurance, the risk of selection bias is notably reduced. However, this strength is somewhat constrained by significant limitations. First, the absence of information on the causes of infertility, body mass index, ART methods, protocol types, endometrial response, and embryonic stages, except for patient age and past medical history, is a significant limitation. These factors are important as they may directly or indirectly affect obstetric and perinatal outcomes. However, the NHIS database is based on claim data, which restricts the availability of detailed clinical information. Additionally, it is not possible to verify the controlled ovarian hyperstimulation protocol or developmental stage of embryo used at the time of ET, which presents another significant limitation. Furthermore, the absence of data on luteal phase support is notable, as progesterone supplements are non-reimbursable medications in the Republic of Korea, limiting access to prescription records beyond those covered by insurance in the national health registry system. Finally, the retrospective nature of the study also serves as a limitation. The quest to determine the most effective method for preparing the endometrium in FET cycles is gaining significance owing to rising global trends in embryo cryopreservation. Mounting evidence suggests a potential link between endometrial preparation methods and obstetric as well as perinatal outcomes. Our study aligns with previous research by demonstrating increased adverse obstetric and perinatal outcomes with AC-FET compared to NC-FET. Consequently, NC-FET emerges as a valuable option to consider from the standpoint of maternal and fetal safety. In cases where NC-FET is not feasible, SC-FET may be preferred over AC-FET, serving as a favorable alternative. SC-FET exhibits lower miscarriage rates than NC-FET and superior obstetric outcomes compared to AC-FET. Therefore, it is essential to consider not only reproductive outcomes but also long-term outcomes such as obstetric and perinatal outcomes when selecting the optimal method for endometrial preparation in FET.