Paternal body mass index is associated with decreased blastocyst development and reduced live birth rates following assisted reproductive technology.

Objective: To determine the relationship between paternal body mass index (BMI), embryo development and pregnancy, and live birth outcomes after assisted reproductive technology (ART).

Design: Retrospective analysis of ART cycles.

Setting: Major assisted reproduction center.

Day 4 embryo selection is equal to Day 5 using a new embryo scoring system validated in single embryo transfers.

Background: Single embryo transfers (SETs) require the most viable embryo from a cohort to be selected. Day 4 embryos may provide a selection advantage similar to blastocyst transfer as embryos are replaced post-embryonic genome activation and into the uterus where they would normally reside. However, there is currently no adequate morphological system to assess Day 4 embryos.