Sex ratios at birth after induced abortion

Background — Skewed male:female ratios at birth have been observed among certain immigrant groups. Data on abortion practices that might help to explain these findings are lacking.

Methods — We examined 1 220 933 births to women with up to three consecutive singleton live births between 1993-2012 in Ontario, Canada. Records of live births, induced and spontaneous abortions were linked to Canadian immigration records. We determined associations of male:female ratios with maternal birthplace, sex of the previous living sibling(s), and prior spontaneous or induced abortions.

Results — Among Canadian-born women and most foreign-born women male:female ratios of infants did not appreciably depart from the normal range, irrespective of the sex of previous children, or the characteristics of prior abortions. However, among immigrant women from India with two prior girls, the overall male:female ratio was 1.96 (95% confidence interval (CI) 1.75-2.21) for the third live birth. Relative to no preceding abortion, the male:female ratio after two girls was 1.77 (95% CI 1.26-2.47) times higher if the current birth was preceded by 1 induced abortion, 2.38 (95% CI 1.44-3.94) times higher if preceded by ≥2 induced abortions, and 3.88 (95% CI 2.02-7.50) times higher if the induced abortion was performed at ≥ 15 weeks gestation. Spontaneous abortions were not associated with son-biased sex ratios in subsequent births.

Interpretation — High male:female ratios observed among infants of Indian-born immigrants are associated with having had induced abortions, especially in the second trimester of pregnancy when fetal sex can be accurately determined by ultrasonography.