Association of secondary sex ratio with smoking and parity

Objective: To assess the sex ratio in offspring of smoking and nonsmoking mothers in relationship to parity. Design: Prospective study. Setting: University hospital. Patient(s): The authors studied 2,108 term singleton neonates born between 1993 and 2002, 665 from smoking mothers and 1,443 from nonsmoking mothers. Intervention(s): A prospective recording of maternal age, parity and smoking status, and gender of neonates delivered over a 10-year period. Main Outcome Measure(s): Secondary sex ratio in regard to maternal smoking and parity. Result(s): The offspring sex ratio in the total sample studied was 1.09; in the offspring of smoking and nonsmoking mothers, it was 1.26 and 1.03, respectively, a statistically significant difference. In the offspring of smoking women who had parity 1, 2, and >= 3, it was 1.47, 1.35, and 0.92, whereas in those of nonsmoking women, it was 1.04, 1.00, and 1.03, respectively (the differences of the parity 1 and 2 groups between the offspring of smoking and nonsmoking mothers were statistically significant). Logistic regression analysis showed that the possibility of a boy being delivered by a mother who smoked was significantly greater in primiparous women than in women who had parity 3, independent of the maternal age. Conversely, parity did not affect significantly the sex ratio in the offspring of nonsmoking women. Conclusion(s): The findings suggest that among women who smoked, significantly more male than female offspring are born from primiparous women, whereas women who had parity 3 gave birth to more female offspring; F biparous women give birth to significantly more male offspring, but the offspring sex ratio declined with the number of cigarettes when the mothers smoked >= 10 cigarettes per day.