Placenta praevia and sex ratio at birth.

The ratio of male to female sex among infants born to 391 women with and 114 079 without placenta praevia was analysed by parity. In the women without placenta praevia the sex ratio decreased significantly with increasing parity, whereas in the women with placenta praevia it increased. Overall, the sex ratio was increased among the women with placenta praevia, particularly multiparas. An increase in the sex ratio at birth is associated with insemination early or late in the menstrual cycle, which may result in delayed development and implantation of the blastocyst; this may be a predisposing factor in placenta praevia.