Zygote transfer on day 1 versus cleavage stage embryo transfer on day 3: a prospective randomized trial

Background: It has been reported that pronuclear morphology is related to embryo quality and viability, and that zygote stage embryos might establish pregnancies after being transferred to the uterus. The objective of this study was to investigate whether transferring zygotes on day 1 would result in similar pregnancy rates compared to transferring cleavage stage embryos on day 3 in a prospective randomized trial. Methods: Patients undergoing IVF/ICSI treatments were randomized to either day 1 or day 3 transfers by envelope withdrawal technique. Zygotes were classified as 'pattern 0' and 'non-pattern 0' according to the size and alignment of pronuclei, the number and distribution of nucleoli. The two best zygotes or embryos were transferred on day 1 or day 3 respectively. The primary outcome measure was pregnancy rate. Results: Pregnancy rates were higher in day 3 group (55/131, 42%) when compared to day 1 (34/123, 28%, P=0.024). Similarly, implantation rates were higher in day 3 group (P=0.03). There were more cycles with cryopreservation in the day 1 group (P<0.001). Embryo quality on day 3 was similar between pattern 0 and non-pattern 0 zygotes. Conclusions: Day 3 embryo transfers result in better pregnancy and implantation rates compared to day 1 zygote transfers. The present pronuclei scoring cannot reliably select zygotes for transfer on day 1.