A comparison of pregnancy outcomes between day 3 and day 5/6 embryo transfers: does day of embryo transfer really make a difference?

Purpose: To determine if day of embryo transfer (ET) affects gestational age (GA) and/or birth weight (BW) at a single university fertility center that primarily performs day 5/6 ET.
Methods: Retrospective cohort study of 2392 singleton live births resulting from IVF/ICSI at a single large university fertility center from 2003 to 2012. Patients were stratified by day 3 or day 5/6 ET. Outcome variables included patient age, gravidity, prior miscarriages, prior assisted reproduction technology cycles, number of embryos transferred, number of single ET, infertility diagnosis, neonatal sex, GA at birth, and BW. Subanalyses were performed on subgroups of preterm infants. A comparison was made between the study data and the Society of Assisted Reproductive Technologies (SART) published data.
Results: There was no difference in GA at birth (39 ± 2.1 weeks for day 3 ET, 39 ± 1.9 weeks for day 5/6 ET) or BW between ET groups (3308 ± 568 g for day 3 ET, 3268 ± 543 g for day 5/6 ET). There was also no difference in the number of preterm deliveries (8.5 % for day 3 ET vs. 10.8 % for day 5/6 ET). The day 5/6 ET study data had significantly fewer pre-term deliveries than the SART day 5/6 ET data.
Conclusion: In contrast to published SART data, GA and BW were not influenced by day of ET. Data may be more uniform at a single institution. Day 5/6 ET continues to offer improved pregnancy rates without compromising birth outcomes.