Background: While the ratio of male to female births (sex-ratio at birth [SRB]) in humans is remarkably stable on the population level, there are many families with multiple same-sex offspring.

Objectives: To identify a putative sub-population with skewed SRB and explore potential factors affecting the SRB.

Methods: A retrospective cohort study including 66,054 families with up to nine same-sex offspring evaluated between 2003 and 2015 at Hadassah-Hebrew University Medical Center. Outcome measures were observed prevalence and SRB of families with up to nine same-sex offspring in a single family. Analyses included the effect of parity, month and year of delivery, inter-delivery interval, and presence of a sequence of previous same-sex offspring on the SRB.

Results: The study comprised 193,411 live-born babies with SRB of 1.057 in favor of males. The proportion of SRB in families with up to nine same-sex offspring did not differ from the calculated presumed proportion. Furthermore, none of the tested factors (parity, month and year of delivery, inter-delivery interval, and the sequence of previous same-sex offspring) were significantly associated with SRB.

Conclusions: SRB was not associated with any of the tested demographic characteristics. We could not identify a skew in SRB even in families with up to nine consecutive same sex offspring. This finding suggests that in the majority of the population the chance of a male or female fetus in each pregnancy remains similar in every pregnancy, regardless of any of the tested variables.