Blastocyst Transfer

The study was approved by an appropriately constituted ethic committee of the National Taiwan University Hospital (Institutional Review Board Number: 201507061RINB), and the written informed consents were obtained from all participants. This study was a retrospective cohort analysis involving women with an indication for COS in IVF or oocyte donation programs between January 2013 and September 2014. All patients were recruited from the Outpatient Department of the Stork Fertility Center (Hsinchu, Taiwan), and were counseled by fertility specialists regarding the stimulation protocol design before treatment began. A single dose of corifollitropin alfa followed by short-acting gonadotropin supplements was administered. Serum hormone levels (luteinizing hormone [LH], oestrodial [E2], progesterone [P4]) and stimulated follicle size were monitored during the follicular phase. Flexible GnRH antagonist administration based on the monitoring serum LH levels was used in the patients. Every patient was treated with corifollitropin alfa in only one IVF cycle. Patients with multiple IVF cycles were not included in this study. The harvested blastocysts were cryopreserved. All of the patients went through frozen-thawed blastocyst transfer(s) in the other menstrual cycle(s) to decrease the OHSS risk and to increase the success rate by optimizing the endometrial synchronization [14 ].

For PPOS protocols, whole embryos were frozen. For the flexible GnRH antagonist group, three days after oocyte retrieval, one or two embryos were transferred under monitoring by abdominal ultrasound or single blastocyst transfer was carried out five days after oocyte retrieval. For cases with severe ovarian hyperstimulation syndrome (OHSS), an endometrial thickness ≤7 mm, progesterone levels ≥2 ng/ml on the hCG trigger day and the presence of uterine fluid, we canceled the fresh embryo transfer, cryopreserved all embryos and subsequently performed frozen embryo transfer (FET). Endometrial preparation for FET was performed by means of the natural cycle for women with regular menstrual cycles and spontaneous ovulation; artificial/induced ovulation cycle for women with irregular menstrual cycles; and downregulation + an artificial cycle for women with endometriosis. Follicle and endometrial scanning was performed by vaginal ultrasound, and embryo or blastocyst transfer was performed using abdominal ultrasound after 3 or 5 days of endometrial development with luteosterone. Routine corpus luteum support, namely, oral dydrogesterone (2 times daily, 10 mg once) (Abbott Co. America) and intravaginal administration of 90 mg of a progesterone sustained-release vaginal gel (Merck Co. Germany), was given. Corpus luteum support was continued at least until 55 days after transfer if pregnancy occurred.

We included women who had at least one singleton live birth resulting from a fresh embryo transferred following IVF (including ICSI) treatment in the UK between 2000 and 2017. Babies born following frozen-thawed ET were excluded from the study since day of ET was not available for frozen cycles in the HFEA dataset. As blastocyst stage transfers were infrequent before 2000, we restricted our sample to women treated between 2000 and 2017. We included live born infants whose gestational age was 22 weeks or more, with a minimum birthweight of 500 g. We excluded still births, births in women under 18 years or over 50 years of age, and those involving oocyte donation, embryo donation, preimplantation genetic testing, or surrogacy. Cycles where more than three embryos were transferred were excluded as many of these resulted in triplet and quadruplet births. Births resulting from ETs on Day 6 were excluded as these only involved frozen embryos.

The unit of analysis here was a singleton live birth episode resulting from transfer of a fresh blastocyst or cleavage stage ET. As some women had two or more singleton live birth episodes arising from several ETs within the study period, all analyses were conducted under a multilevel framework, which accommodated repeated cycles resulting in livebirths within the same women. In order to account for the dependency between cycles resulting in live birth within women, a population-averaged model using generalized estimating equations was used to explore associations between the exposure groups (blastocyst versus cleavage stage ET) and perinatal outcomes (Hardin and Hilbe, 2003 ) and to estimate 95% CI using robust standard errors that allowed for correlation within women (McCullagh and Nelder, 1989 ). We specified an exchangeable correlation structure, which assumes that the risk of a perinatal adverse event was the same for any live birth within a woman. For the outcomes of preterm birth (preterm birth versus full-term birth), congenital anomaly (yes versus no), and healthy baby status (yes versus no), a robust Poisson regression model was used. For the two birthweight outcome variables (i.e. birthweight coded as low, normal or high, and birthweight adjusted for gestational age coded as SGA, AGA, or LGA), a multinomial logistic regression model was employed since each of these variables had three categories (Chamberlain, 1980 ; Pforr, 2014 ). The association between treatment strategy (blastocyst or cleavage stage ET) and very preterm birth (versus full-term birth) was estimated using multinomial logistic regression (where we also included 32–37 weeks gestation as a nuisance outcome category). Crude risk ratios (RRs), adjusted RRs (aRRs), and 95% CI were calculated. The following factors were considered as confounders: maternal age (years), cause of infertility (i.e. tubal disease, ovulatory disorder, male factor, unexplained), previous pregnancy status (yes/no), treatment type or type of insemination (IVF versus ICSI), number of eggs collected, and year of treatment. The covariates considered for adjustment differed for each of the outcomes and are listed in the footnote under each table. Since ET stage could influence birthweight through its effect on gestational age, gestational age can be considered to be a mediator on the causal pathway from cleavage or blastocyst stage ET to birthweight. Therefore, it was excluded to avoid bias since its inclusion does not allow us to estimate the total direct effect of the stage of ET on birthweight (Wilcox et al., 2011 (link)). In the same way, the number of embryos transferred was considered as a mediator and was excluded from multivariable analyses. Further, congenital anomalies or the underlying cause of congenital anomalies have been linked with iatrogenic preterm birth owing to early induction of labour (Brown, 2009 (link)). In this case, gestational age would be considered a collider rather than a confounder as both ET stage and congenital anomaly can affect gestational age through independent routes. Therefore, gestational age was also excluded from this analysis.

The study was reviewed and approved by the Ethics Committee of Nanjing Maternity and Child Health Care Hospital according to the Declaration of Helsinki (2022KY-049), conducting at the reproductive center and the obstetric department of the same hospital. The delivery timing spanned from January 2019 to March 2022. The inclusion criteria were as follows: (a) single D5 or D6 blastocyst transfer; (b) first or second FET transfer; (c) autologous oocytes; and (d) singleton delivery at ≥ 28 weeks of gestation. The exclusion criteria were as follows: (a) uterine malformation; (b) delivery at other hospitals; (c) identical twins; (d) stillbirth (fetal death after 28 weeks of pregnancy); and (e) missed cycle data.