Sex ratio is remarkably constant

Objective: To study whether the sex of the offspring is related to increasing parental age, gravidity, and parity, hypothesizing an altered male-to-female sex ratio with the advancing parental age. Design: A large retrospective cohort study. Setting: The study analyzed birth records of Hadassah Hebrew University Medical Center in Jerusalem from June 2003 to December 2006. Patient(s): 35,837 birth records were analyzed including 941 multifetal deliveries, excluding foreign inhabitants (n = 744), missing data for the main study outcome (n = 2) and parturients over 50 years to control for egg donation (n = 26). Intervention(s): None. Main Outcome Measure(s): Male-to-female sex ratio. Result(s): The male-to-female sex ratio of all the newborns was 1.05. This ratio did not change significantly with either maternal or paternal age. Neither gravidity nor parity affected the male-to-female ratio. The only factor that affected the regression of sex ratio was the length of gestation. Conclusion(s): Sex ratio at birth is remarkably constant. No association was found between parental age or birth order and neonatal sex ratio.