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Purpose: This study aimed to compare slow freezing (SF) and vitrification (VT) techniques for day 3 embryo cryopreservation in infertile couples., Methods: This retrospective cohort study enrolled 5613 infertile patients, with 7862 frozen-thawed day 3 embryos and 3845 vitrified-warmed day 3 embryos, from 2010 to 2014, at a single center. The rates of embryo survival, pregnancy, implantation, miscarriage, live birth, and live birth weight were compared between the two groups., Results: A total of 5613 cycles with 5520 transfers were analyzed. Using SF, the rates of overall embryo survival and fully intact blastomeres were lower than those in VT (91.5 vs. 97.4 % and 68.7 vs. 92.3 %, respectively). The rate of good quality embryos after thawing/warming was lower in SF than in VT. In single frozen embryo transfer cycles (FETs), the pregnancy and implantation rates were similar between the two groups (35.0 vs. 40.8 % and 34.6 vs. 35.9 %, respectively). In double FETs, the pregnancy rate per cycle was also similar between the groups (58.8 vs. 58.4 %). The implantation rate per embryo transfer was significantly higher with SF than with VT (38.8 vs. 34.6 %). With adjustment for maternal age and the number of good quality embryos, differences in implantation rate remained significant (adjusted P value, SF vs. VT P < 0.05). No independent effect was found for the method of cryopreservation on the pregnancy rate. No significant differences in the rates of miscarriage, live birth, and live birth weight were observed between the two techniques., Conclusions: Despite the significantly low embryo survival rate, fully intact blastomere rate, and good quality embryo rate in SF, the pregnancy and implantation rates were not adversely affected in single and double FETs. SF yielded an equivalent miscarriage rate, live birth rate, and live birth weight compared with VT. The SF protocol to cryopreserve day 3 embryos still should be considered.

Objective: To explore the risk factors associated with twin pregnancy in double embryo transfer., Methods: A retrospective analysis was performed for 2970 double embryo transfer cycles, including 1984 cycles of fresh embryo transfer and 986 cycles of frozen-thawed embryo transfer (FET). Multiple factor Logistic regression was used. The twin pregnancy rate was studied in group of age < 35 or age ≥ 35 years old, in group of zero, single or double high-quality embryos transferred and group of fresh embryo transfer or FET cycles., Results: (1) There was a significantly negative correlation between women's age and twin pregnancy according to the analysis of multiple factor Logistic regression (P < 0.01). And a significantly positive correlation existed between the number of high-quality embryos transferred, FET and twin pregnancy (P < 0.01); (2) the twin pregnancy rate of both fresh embryo transfer and FET cycles was higher in group of age < 35 years than that in group of age ≥ 35 years (16.0% vs 8.0%, P < 0.01; 26.9% vs 14.2%, P < 0.01); (3) the twin pregnancy rate of fresh embryo transfer cycles with double high-quality embryos transferred was higher than that with zero or single high-quality embryos transferred (19.1% vs 5.4%; 19.1% vs 11.0%, P < 0.01); the twin pregnancy rate of FET cycles with double high-quality embryos transferred was also higher than that with zero or single high-quality embryos transferred (32.7% vs 10.8%; 32.7% vs 20.7%, P < 0.01); (4) the twin pregnancy rate of FET cycles was significantly higher than that of fresh embryo transfer cycles (24.7% vs 14.9%, P < 0.01)., Conclusion: Women's age, the number of high quality embryos transferred and FET are risk factors associated with twin pregnancy. At reproductive centers with a mature technique of FET, we recommend performing single high-quality embryo transfer in FET cycles to reduce the occurrence of twin pregnancy.

Background: Frozen-thawed embryo transfer (FET) is the most common way to prevent serious late ovarian hyperstimulation syndrome and increase the cumulative pregnancy rate. We evaluated the effectiveness of an FET program for improving the embryo implantation and clinical pregnancy rates, and ultimate embryo utilization rate in infertility treatment., Methods: Patients undergoing in vitro fertilisation (IVF) cycles from January 2006 to June 2008 were enrolled, including 179 patients who had undergone the first FET cycle after controlled ovarian hyperstimulation (COH) in which all embryos were frozen (group C1) and 1306 patients who had COH with fresh embryo transfer (ET) (group T1). Logistic regression was used to model the embryo implantation and clinical pregnancy rates based on the mother's age, numbers of oocytes retrieved, embryos transferred and high-quality embryos transferred. The embryo implantation and clinical pregnancy rates were also compared between two groups after adjusting for age, the numbers of oocytes retrieved and the numbers of embryos transferred., Results: Logistic regression analysis confirmed that embryo implantation and clinical pregnancy rates in group C1 were both significantly higher than those in group T1 after adjusting for confounding factors (43.6% vs 29.0%, 63.1% vs 47.0%, respectively; P < 0.01). The embryo implantation and clinical pregnancy rates were consistently higher in group C1 by comparing the age groups >or= 35 or < 35 years. The clinical pregnancy rates for the numbers of oocytes retrieved per cycle being >or= 15 or < 15 were higher in group C1, as was the embryo implantation rate. These differences were statistically significant for oocyte numbers >or= 15 (P < 0.05). The embryo implantation and clinical pregnancy rates in group C1 were both significantly higher than in group T1 when two or three embryos were transferred (P < 0.05)., Conclusion: A program of freezing all embryos and performing FET improved the rates of embryo implantation and clinical pregnancy, and ultimately enhanced the embryo utilization rate.