Endocrinology: The sex of the baby may be related to the length of the follicular phase in the conception cycle

In a prospective study of normal couples discontinuing contraceptionto begin a pregnancy, the days of ovulation were estimated byhormonal assay of daily urine specimens. No hormonal interventionswere used. Length of the follicular phase (from onset of mensesto ovulation) was found to be related to the sex of the babyamong 133 pregnancies that survived to delivery. Conceptioncycles with short follicular phases (early ovulation) tendedto produce boys, while those with long follicular phases tendedto produce girls. This relationship is consistent with otherdata and could explain sex-related differences in the lengthof gestation and the observed excess of same-sex pairs amongdizygotic twins.