Concerns of possible male biased sex ratios with IVF exist due to reports of faster preimplantation development for male relative to female embryos in a variety of mammals, including humans. This study reports the first direct test of the hypothesis that selection for faster preimplantation development to the blastocyst stage increases the likelihood of a male birth. Gender outcomes of all live births resulting from blastocyst stage embryo transfer within a large assisted reproduction practice were reviewed, along with associated embryological records. The developmental stage of transferred embryos relative to the average developmental stage of all viable embryos in each cohort was related to gender outcomes by logistic regression analysis. Gender data were available for 1,184 children born from 815 pregnancies resulting from blastocyst stage embryo transfer during the study period. Data on preimplantation embryonic development were available for 737 cycles resulting in the birth of 1,075 children. There was no relationship between selection for developmental rate and the gender of resulting children. Numbers of male and female children were nearly identical within this patient population (593 male versus 591 female). These results indicate that offspring gender is unrelated to selection for developmental rate among blastocyst transfer cycles.

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