Influencing factors of live birth in vitrified oocyte donation cycles: A retrospective cohort study.

In accordance with Chinese regulations and hospital standards, oocyte donors met all of the following criteria: (i) undergoing their first IVF cycle with more than 20 oocytes retrieved; (ii) aged 20–35 years; (iii) normal karyotype; (iv) no more than two spontaneous miscarriages; (v) negative results for infectious disease screening; (vi) no family history of hereditary or chromosomal diseases. Recipients were eligible if they met at least one of the following indications: (i) ovarian failure; (ii) carrying significant genetic defects; (iii) history of poor oocyte or embryo quality. Donors and recipients remained mutually anonymous. Each recipient receives oocytes solely from one single oocyte donor. All donated oocytes from the same donor are thawed at one time, and oocytes from one donor are only allocated to a single recipient.

Yangfeng Yu: Writing – review & editing, Investigation. Ge Lin: Writing – review & editing, Supervision, Resources, Funding acquisition, Conceptualization. Fei Gong: Writing – review & editing, Supervision, Resources, Project administration, Conceptualization. Qiuyun Yan: Writing – review & editing, Investigation. Juan Song: Writing – review & editing, Investigation. Xiaojuan Wang: Writing – original draft, Project administration, Methodology, Formal analysis, Conceptualization. Shuoping Zhang: Writing – review & editing, Resources, Project administration, Investigation, Funding acquisition, Data curation, Conceptualization.

This study was approved by the Ethics Committee of the Reproductive and Genetic Hospital of CITIC-Xiangya, Changsha, People’s Republic of China (approval number, LL-SC−2025–013), and followed the Strengthening the Reporting of Observational Studies in Epidemiology reporting guideline. All donors provided written informed consent for the cryopreservation of a certain number of oocytes after oocyte retrieval. Subsequently, after they had used their own oocytes to generate multiple high-quality blastocysts, achieved a successful pregnancy, or delivered a healthy baby, they were advised to sign an informed consent form for donating their surplus frozen oocytes.

The study was supported by Hunan Provincial Grant for Innovative Province Construction ( 2019SK4012 ), 10.13039/501100001809 National Natural Science Foundation of China ( 22374146 ) and National Natural Science Foundation of China ( 62373076 ).

The primary outcome was live birth of the first embryo transfer, defined as the delivery of a viable infant after the first embryo transfer. The secondary outcome was cumulative live birth, defined as at least one live birth per initiated oocyte donation cycle, which included all cycles utilizing fresh and/or frozen embryo transfer until a live birth occurred.

Between January 1, 2015 and June 30, 2024, a total of 380 infertile patients underwent thawing of vitrified donated oocytes. Three patients who underwent PGT were excluded, and 377 women were included in the final analysis. Among them, eight patients had no usable embryos, and 369 patients underwent the first embryo transfer, resulting in 190 live births and 179 non-live-birth cases. The live birth rate of the first embryo transfer cycle was 51.5% (190/369). The baseline and treatment characteristics of the live-birth and no-live-birth groups were presented in Table 1 . Table 1 Demographic and clinical characteristics of recipients according to live birth outcome. Overall (n = 377) No-live birth (n = 187) Live birth (n = 190) P Recipient age, years, mean ± SD 38.15 ± 6.45 38.88 ± 6.52 37.43 ± 6.31 0.028 Recipient BMI, kg/m 2 , mean ±SD 21.34 ± 1.78 21.36 ± 1.91 21.33 ± 1.65 0.867 Infertility years, years, mean ±SD 8.00 ± 4.93 7.90 ± 5.02 8.09 ± 4.85 0.707 Infertility type, n(%) 0.471 Primary 191(50.8) 91(48.7) 100(52.9) Secondary 185(49.2) 96(51.3) 89(47.1) Endometriosis, n(%) 1.000 No 328(87.0) 163(87.2) 165(86.8) Yes 49(13.0) 24(12.8) 25(13.2) Male age, years, mean (SD) 40.16 ± 7.03 40.66 ± 7.03 39.67 ± 7.02 0.172 Severe oligoasthenospermia, n(%) 0.343 Yes 34(9.0) 20(10.7) 14(7.4) No 343(91.0) 167(89.3) 176(92.6) Indication for donor oocyte use, n(%) 0.040 Poor ovarian reserve 268(71.1) 123(65.8) 145(76.3) Poor oocyte/embryo quality 46(12.2) 30(16.0) 16(8.4) Genetic factor 63(16.7) 34(18.2) 29(15.3) Duration of oocyte cryopreservation, months, mean ±SD 29.47 ± 15.24 29.38 ± 15.83 29.57 ± 14.68 0.905 Number of thawed oocytes, mean ±SD 5.68 ± 1.17 5.70 ± 1.16 5.66 ± 1.18 0.757 Number of day3 usable embryos, mean ±SD 3.62 ± 1.53 3.40 ± 1.68 3.83 ± 1.33 0.006 Number of transferred embryos, n(%) 0.004 0 8(2.1) 8(4.3) 0(0.0) 1 25(6.6) 16(8.6) 9(4.7) 2 344(91.3) 163(87.2) 181(95.3) Type of transferred embryos, n(%) 0.298 Cleavage stage 349(94.3) 167(92.8) 182(95.8) Blastocyst stage 21(5.7) 13(7.2) 8(4.2) Endometrial preparation protocol, n(%) 0.206 Natural cycle 63(16.7) 32(17.1) 31(16.3) Artificial cycle 156(41.4) 71(38.0) 85(44.7) Down-regulation artificial cycle 92(24.4) 54(28.9) 38(20.0) Others 66(17.5) 30(16.0) 36(19.0) Endometrial thickness before embryo transfer, mm, mean ±SD 11.36 ± 1.33 11.28 ± 1.40 11.45 ± 1.25 0.228 Demographic and clinical characteristics of recipients according to live birth outcome.

A retrospective cohort study was conducted at the Reproductive and Genetic Hospital of CITIC-Xiangya. All patients who underwent ART using vitrified donated oocytes between January 1, 2015 and June 30, 2024 were enrolled. The follow-up period ended on June 30, 2025, and was terminated early if a live birth occurred or no embryos remained for transfer. Patients who underwent preimplantation genetic testing (PGT) were excluded from the analysis.

This study demonstrated that advanced recipient age was associated with a reduced LBR in ART using vitrified donated oocytes, which is consistent with previous studies [10] , [11] . Although oocyte donation provides young and high-quality oocytes, advanced maternal age may impair endometrial receptivity, leading to lower implantation and live birth rates [17] . A notable finding is that the decline in LBR with age was gradual, and recipients aged 45 years and older still achieved a clinically meaningful LBR, suggesting that oocyte donation remains a feasible and effective option for older women. Recipients who received donated oocytes due to a history of poor oocyte/embryo quality had a significantly lower live birth rate (LBR) than those who received donated oocytes for other indications. As shown in Table 3 , further subgroup analysis revealed that the significantly reduced LBR in this group was mainly driven by patients with a history of poor embryo quality, specifically those with poor-quality cleavage-stage embryos and blastocyst formation defect, who had extremely low live birth rates. Since oocyte donors were strictly selected and the quality of donated oocytes can be assumed to be consistent, the lower LBR in this subgroup was likely attributed to the effect of sperm quality on embryonic development [18] . Given that the ICSI technology allowed technicians to select sperm, it had largely reduced the impact of the routine concentration and morphology of sperm [19] , [20] , [21] on the pregnancy outcome. This study had not yet found that severe oligoasthenospermia affected the LBR. This implied that intrinsic sperm quality factors, such as sperm DNA integrity and epigenetics, may play a role [21] , [22] . Therefore, this subgroup of patients with a history of poor embryo quality may benefit from interventions aimed at improving sperm quality, such as antioxidant therapy, or advanced sperm selection techniques to identify sperm with better developmental potential. This study found that a higher number of day−3 usable embryos was associated with an increased CLBR, because more usable embryos provide more opportunities for subsequent embryo transfers. The relatively modest increase in CLBR in this study may be explained by the low rate of second embryo transfers (only 42.7% of patients with failed first transfer underwent a second transfer). The number of thawed oocytes was not associated with LBR or CLBR, possibly because 90% of recipients received no more than 6 oocytes, limiting the analytical range. However, increasing the number of thawed oocytes significantly increased the number of surplus embryos after the first live birth. Given the scarcity of donated oocytes in China, a batch allocation strategy is advisable when six or more oocytes are available, thereby expanding access to donated oocytes for a greater number of patients. Recipient BMI was not associated with LBR in this study, which may be due to the fact that 89.7% of recipients had a normal BMI, resulting in limited statistical power to detect potential effects. Oocyte cryopreservation duration, up to 87 months, showed no adverse impact on laboratory or clinical outcomes, supporting the long-term safety and stability of oocyte vitrification [8] , [9] , [23] . Physicians and patients can be reassured that prolonged storage does not compromise oocyte developmental competence or pregnancy outcomes. This study is a large-scale real-world study focusing on the Chinese population undergoing ART with vitrified donated oocytes under national regulations. The results of this study can provide important reference for the formulation of guidelines for oocyte donation in the local area. It has several limitations. First, due to local ethical and legal requirements mandating the anonymization of oocyte donor information, we were unable to adjust for donor-specific characteristics. However, only individuals meeting stringent donor criteria were eligible to become oocyte donors, and donors inherently represent a prognostically optimal population, which substantially reduces bias introduced by donor heterogeneity. Second, only a small number of recipients underwent sperm DNA integrity testing, so we could only speculate rather than confirm the role of sperm quality in the lower LBR among patients with poor embryo history. Finally, this was a single-center study, which may limit the generalizability of the results. Nevertheless, the standardized screening criteria and treatment protocols ensure the reliability and accuracy of the findings.

All donated cumulus-oocyte complexes were cultured for 2–3 h before denudation and vitrification. Only metaphase II (MII) oocytes were vitrified using the Kitazato vitrification kit and Cryotop open straw, following the protocol described by Wang et al. [15] . One to two oocytes were vitrified in each straw. After thawing, oocytes were cultured for 2–3 h before intracytoplasmic sperm injection (ICSI). Oocyte survival was evaluated based on cytoplasmic morphology and membrane integrity. Normal fertilization was confirmed by the presence of two pronuclei (2PN) 16–18 h after insemination. Embryos were cultured in G1.5 medium (Vitrolife) in a tri-gas incubator (ASTEC APM−50D, Japan). On day 3, embryos were evaluated for cleavage and graded according to Puissant’s criterion. Day−3 usable embryos were defined as those with four or more cells and a fragmentation rate lower than 50%. High-quality embryos were defined as those with ≥ 6 cells and a fragmentation rate < 20%. If the patient had high-quality embryos, day 3 transfer was recommended, and the remaining embryos were cryopreserved. If the patient did not have high-quality embryos, it was recommended that the embryos underwent blastocyst culture followed by blastocyst transfer. Blastocysts were graded using the Gardner and Schoolcraft system on days 5–7 [16] . A maximum of two high-grade embryos were transferred into the recipient’s uterus, and surplus usable embryos were vitrified for subsequent transfers.

This study comprehensively analyzed clinical outcomes and influencing factors of vitrified donated oocyte ART in Chinese patients. Advanced recipient age was associated with a gradual reduction in LBR. A history of poor oocyte/embryo quality was an independent risk factor for lower LBR. An increased number of day−3 usable embryos significantly improved CLBR. Oocyte vitrification is safe and not adversely affected by long-term storage. To alleviate the shortage of donated oocytes, a batch allocation strategy when six or more oocytes are available is recommended to improve utilization efficiency and expand access for more infertile families.

A conventional artificial cycle was typically used for endometrial preparation; however, if the patient had conditions such as adenomyosis or endometritis, a down-regulation artificial cycle was used. During the artificial cycle, oral estradiol was given in fixed doses or in a stepwise increasing manner for about 10 days. Progesterone was supplemented to facilitate endometrial transformation when endometrial thickness ≥ 8.5 mm and serum progesterone < 1 ng/mL. Embryo transfer was arranged on day 4 of luteal support for cleavage embryos and day 6 for blastocysts.

Among 369 patients who underwent the first embryo transfer, 80 received a second transfer and 13 received a third transfer after previous failures. The cumulative live birth rate (CLBR) was 58.5% (216/369) (see Supplementary Figure 3 ). The analysis of influencing factors for cumulative live birth (as shown in Table 4 ) Table 4 Logistic regression analysis of cumulative live birth. Estimate OR (95%CI) P Recipient age −0.072 0.93(0.87,1.00) 0.041 Recipient BMI 0.038 1.04(0.92,1.18) 0.557 Infertility years 0.013 1.01(0.97,1.06) 0.582 Infertility type 0.288 1.33(0.77,2.32) 0.302 Endometriosis 0.186 1.20(0.62,2.37) 0.585 Male age 0.019 1.02(0.96,1.08) 0.543 Severe oligoasthenospermia −0.054 0.95(0.44,2.05) 0.890 Indication for donor oocyte use Poor ovarian reserve Ref Poor oocyte/embryo quality −0.845 0.43(0.22,0.82) 0.011 Genetic factor −0.362 0.70(0.37,1.33) 0.271 Duration of oocyte cryopreservation 0.004 1.00(0.99,1.02) 0.557 Number of thawed oocytes −0.104 0.90(0.74,1.10) 0.305 Number of day−3 usable embryos 0.388 1.47(1.26,1.73) 0.000 Logistic regression analysis of cumulative live birth. showed that advanced recipient age reduced CLBR (OR: 0.93; 95%CI: 0.87–1.00; P  = 0.041). A history of poor oocyte/embryo quality also decreased CLBR (OR: 0.43; 95%CI: 0.22–0.82; P  = 0.011). In addition, an increased number of day−3 usable embryos significantly improved CLBR (OR: 1.47; 95%CI: 1.26–1.73; P  = 0.000). Analysis of remaining embryos after the first live birth showed that increasing the number of thawed oocytes did not significantly improve LBR of the first embryo transfer or CLBR, but markedly increased the number of surplus embryos ( P for linear trend = 0.000). When the number of thawed oocytes reached 6, the average number of remaining embryos after the first live birth was 1.50 (see Supplementary Figure 4 ).

Data were presented as mean ± standard deviation (SD) or frequency (%). Normality of continuous variables was tested using the Shapiro-Wilk test. Differences between groups were analyzed using two-sample t -test, Kruskal-Wallis test, or Pearson chi-square test, as appropriate. Multivariate logistic regression was used to identify independent factors associated with live birth after the first embryo transfer and cumulative live birth. Line charts were used to illustrate linear trends of live birth rate (LBR) with recipient age and oocyte cryopreservation duration. Linear trends were tested using linear regression models. The nonlinear relationship between cryopreservation duration and live birth was analyzed using restricted cubic spline (RCS) models based on logistic regression.

Approximately 15% of women of reproductive age experience infertility, which has become a significant global public health issue [1] . For women who may not be able to conceive due to diminished ovarian reserve, carrying significant genetic defects, or repeated poor embryo quality, oocyte donation-assisted reproductive technology (ART) offers a valuable opportunity to achieve pregnancy and build a family [2] . Regulations and clinical practice of oocyte donation vary widely across different countries and regions. In China [3] , oocyte donors are limited to women aged 20–35 years who are undergoing IVF or ICSI treatment. Surplus oocytes can be donated only when the number of oocytes reserved for personal use reaches 15, and strict anonymity between donors and recipients is required by law. These regulations effectively prevent commercialization of oocyte donation and protect the rights and safety of both donors and recipients. However, they also lead to an extreme shortage of donated oocytes. According to the Chinese Society of Reproductive Medicine, oocyte donation cycles account for only 0.25% of all ART cycles in China, while the global average is approximately 9% [4] . Previous surveys have shown that approximately 85–95% of women who need donated oocytes for ART treatment are unable to obtain them in China [3] . Even in Europe, where donor sources are more diverse, a notable shortage of donated oocytes still exists [5] . Therefore, optimizing oocyte donation strategies and improving the success rate of ART using donated oocytes have become critical to improving oocyte utilization efficiency and helping more infertile families achieve their childbearing goals. Since the widespread application of vitrification in oocyte cryopreservation, vitrified oocyte donation has greatly improved the flexibility and efficiency of oocyte donation programs. Previous studies have compared live birth outcomes between fresh and frozen oocyte donation cycles [6] , and evaluated the effects of different cryopreservation techniques (slow-freezing vs. vitrification) [7] , cryopreservation duration [8] , [9] , and other technical factors on laboratory and clinical outcomes. Most studies have confirmed that vitrification is a safe and effective method for oocyte cryopreservation. Other studies have explored the impacts of donor and recipient age [10] , [11] and body mass index (BMI) [12] , [13] on live birth rates. However, most of these studies only focused on individual factors and adjusted for a limited number of confounding variables, making it difficult to conduct a comprehensive assessment of factors influencing live birth. Fitzgerald et al. [14] developed a prediction model for live birth in oocyte donation cycles based on Australian population data. Nevertheless, oocyte donation regulations in Australia differ substantially from those in China, including donor age limits, donor sources, oocyte retention requirements, and directed donation policies. These differences result in distinct donor and recipient characteristics, limiting the generalizability of those findings to Chinese clinical practice. This study aimed to evaluate the clinical outcomes of patients undergoing ART with vitrified donated oocytes under Chinese regulatory and clinical settings. We analyzed multiple factors associated with live birth, including donated oocyte characteristics, recipient conditions, and male partner factors. The purpose was to provide evidence for optimizing oocyte donation strategies and helping more infertile families achieve successful childbirth.

The author(s) declare(s) that they have no competing interests.