Abstract
Despite the large number of studies on blastocyst transfers, it is unclear whether day 6 blastocysts have similar pregnancy rates and safety with day 5 blastocysts. Thus, this study aimed to compare the obstetric, neonatal, and clinical outcomes of day 5 and day 6 vitrified blastocyst transfers (VBT). In this retrospective cohort study with propensity score matching, we evaluated 1,313 cycles of VBT performed between January 2014 and December 2015 at the Fertility Center of CHA Gangnam Medical Center. All cycles underwent natural endometrial preparation. We used propensity score matching to compare day 5 and day 6 VBTs in a matched comparison. After propensity score matching, there were 465 cycles of day 5 VBT and 155 cycles of day 6 VBT. Implantation rate (IR), clinical pregnancy rate (CPR), and live birth rate (LBR) were significantly lower in day 6 VBTs (44.2 vs. 53.1%, p = 0.023; 48.4 vs. 60.4%, p = 0.009; 33.5 vs. 51.8%, p < 0.001). Miscarriage rate was significantly higher in day 6 VBTs (29.3 vs. 10.7%, p < 0.001). Rate of multiple gestations was similar between the two groups (29.3 vs. 30.2%, p = 0.816). Assessing 241 and 52 babies from day 5 and day 6 VBTs, no differences were found in neonatal outcomes including rates of low birth weight, preterm birth, and congenital malformations. In propensity score-matched analysis, obstetric, and neonatal outcomes between day 5 and day 6 VBTs were similar so that day 6 VBTs are as safe as day 5 VBTs. IR, CPR, and LBR were are all significantly lower in day 6 VBTs. Therefore, if there are no differences in the morphological grade between day 5 and day 6 blastocysts, transfer of day 5 vitrified blastocysts should be considered first.
Highlights
As techniques for in vitro fertilization (IVF) and embryo culture have become advanced, many IVF centers can transfer embryos at the blastocyst stage
The number of 2PN zygote decreased in the day 6 group (p = 0.048), and the number of freezing blastocysts was lower in the day 6 group (p = 0.007)
Our present result indicates that vitrified blastocyst transfers (VBT) cycles with day 6 blastocysts were significantly inferior to those with day 5 blastocysts in terms of Implantation rate (IR), clinical pregnancy rate (CPR), and live birth rate (LBR)
Summary
As techniques for in vitro fertilization (IVF) and embryo culture have become advanced, many IVF centers can transfer embryos at the blastocyst stage. Compared with normally growing embryos, there were increased number of abnormal mitotic spindle [2], decreased expression of mitotic spindle [3], and more molecular abnormalities [4] in growth-retarded embryos These phenomena have raised the question: Does blastocysts with a delayed blastulation maintain acceptable pregnancy rates with safety?. To answer this question, several studies have compared IVF outcomes of day 5 and day 6 blastocyst embryo transfers. Controlled ovarian hyperstimulation advances endometrial maturation by 1–2.5 days compared with the expected chronological date from oocyte retrieval It causes asynchronous uterine environment with poor endometrial receptivity and may decrease pregnancy rates [9,10,11]. It is difficult to determine whether poor endometrial receptivity is due to impaired embryo quality of day 6 blastocysts or asynchronous uterine environment with poor endometrial receptivity [5, 12]
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