['Our purpose was to assess the possible relationship between human embryo growth rates and sexual differentiation.', 'We analyzed 142 conceptional in vitro fertilization and embryo transfer cycles undertaken in 137 women (151 embryos transferred) in which each embryo transferred led to a gestational sac. Embryos were sorted into three groups according to the number of blastomeres assessed just before embryo transfers < or = 3, 4, and > or = 5 blastomeres.', 'Percentages of girls and boys remained roughly unaltered irrespective of the number of blastomeres observed just before embryo transfer: < or = 3 blastomeres, 45% and 55%; 4 blastomeres, 44% and 56%; and > or = 5 blastomeres, 45% and 55%, respectively (statistical power 90% at the 5% significance level).'] Are female and male human embryo growth rates similar before the eight-cell stage?