STUDY QUESTION Is transferring embryos that achieve blastulation on Day 7 effective and safe? SUMMARY ANSWER Embryos that achieve blastulation on Day 7 resulted in clinically relevant rates of clinical pregnancy (32.5%) and live birth (25.2%), and newborns have a similar risk of low birth weight, congenital malformations or early neonatal death compared with those derived from Days 5 and 6 blastocysts. WHAT IS KNOWN ALREADY Potential advantages of blastocyst transfer over cleavage embryo transfer have led to a shift toward the former in IVF practice. However, published data about the fertility outcomes of transferring embryos with a delayed blastulation on Day 7 are scarce and controversial. Moreover, there are few data available on the neonatal outcomes of Day 7 blastocysts. As a result, the clinical value of Day 7 blastocysts is uncertain. STUDY DESIGN, SIZE, DURATION This was a retrospective cohort study that included 2908 women undergoing frozen-thawed embryo transfer cycles of IVF/ICSI from January 2006 to May 2015, and reported on the 1518 live born infants from those cycles. PARTICIPANTS/MATERIALS, SETTING, METHODS We used propensity score matching to compare the fertility outcomes of women undergoing Day-5, Day-6 and Day-7 vitrified embryo transfers in three matched comparisons (Day 5 vs Day 6, Day 5 vs Day 7 and Day 6 vs Day 7). We also compared neonatal outcomes among babies derived from Day-5, Day-6 and Day-7 vitrified embryo transfers. MAIN RESULTS AND THE ROLE OF CHANCE We studied 922 Day-5, 1752 Day-6 and 234 Day-7 vitrified embryo transfers. Day-7 vitrified embryo transfers had significantly lower implantation, clinical pregnancy and live birth rates than both Day-5 (23.9 vs 49.9%, 31.7 vs 58.1% and 25.1 vs 46.5%, all P < 0.001, respectively) and Day-6 (24.7 vs 42.3%, 33.0 vs 53.2% and 25.6 vs 41.4%, all P < 0.001, respectively) vitrified embryo transfers. Assessment of babies showed no statistically significant difference in the rates of low birth weight, congenital malformations and early neonatal death among the 585, 869 and 64 babies born from Day-5, Day-6 and Day-7 vitrified embryo transfer groups, respectively. LIMITATIONS, REASONS FOR CAUTION This was a single center retrospective study, and most of the neonatal data were extracted from parental questionnaires. Besides, the number of Day-7 vitrified embryo transfer cycles and babies born from these cycles was still limited, thus reducing the power of our study in assessing neonatal outcomes. In addition, only the morphologically poorer Day 3 embryos were extendedly cultured, and poorer blastocysts were qualified for vitrification on Day 7 than on Day 5 or 6, both of which might bias clinical pregnancy rates. WIDER IMPLICATIONS OF THE FINDiNGS Transfer of embryos that reach the blastocyst stage on Day 7 results in lower but still acceptable live birth rate, and seems to be safe for the offspring. Extension of the culture time in embryos that do not reach blastocyst stage by Day 6 should be assessed in randomized clinical trials. STUDY FUNDiNG/COMPETING INTEREST(S) This work was supported by the National Nature Science Foundation of China (Grant nos. 81771533, 81571397, 81571486, 31770989 and 81501319), the Nature Science Foundation of Shanghai (Grant nos. 15ZR1424900 and 1441196300), and the Foundation of Health and Family Planning Commission of Shanghai (Grant no. 201540237). B.W.M is supported by the National Health and Medical Research Council (NHMRC) Practitioner Fellowship (GNT1082548), B.W.M reports consultancy for ObsEva, Merck and Guerbet. TRIAL REGISTRATION NUMBER Not applicable.
- Day 7 blastocyst
- frozen-thawed embryo transfer
- neonatal outcome