['A higher proportion of male offspring has been observed after transferring faster-developing embryos in a number of animal species. Therefore, we evaluated the correlation between the sex ratio of delivered babies and the cleavage stage of transferred embryos in a human IVF-ET program.', 'The sex of infants born (n = 104) after transfer of exclusively slower-cleaving < or = 3 cell (n = 41) versus exclusively faster-cleaving > or = 4 cell (n = 63) embryos was compared. Furthermore, all boys and girls resulting from IVF-ET (n = 213) were compared with respect to: the average number of cells in the embryos that were transferred, the embryo with the greatest number of cells in the cohort transferred and the percentage of embryos that were faster cleaving.', 'Thirty seven percent (15/41) of infants resulting from the transfer of exclusively slower-growing embryos were girls and 38% (24/36) of the infants from the faster-growing embryos were girls (NS). The analysis all 213 babies born after 145 embryo transfer procedures did not suggest any differences in embryo cleavage rates in embryo transfers leading to male versus female infants.'] Is sex ratio of babies unchanged after transfer of fast- versus slow-cleaving embryos?