Sex ratios at birth after induced abortion

Abstract

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 3 consecutive singleton live births between 1993 and 2012 in Ontario. Records of live births, and induced and spontaneous abortions were linked to Canadian immigration records. We determined associations of male:female infant ratios with maternal birthplace, sex of the previous living sibling(s) and prior spontaneous or induced abortions.

Results: Male:female infant ratios did not appreciably depart from the normal range among Canadian-born women and most women born outside of Canada, irrespective of the sex of previous children or the characteristics of prior abortions. However, among infants of women who immigrated from India and had previously given birth to 2 girls, the overall male:female ratio was 1.96 (95% confidence interval [CI] 1.75-2.21) for the third live birth. The male:female infant ratio after 2 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 or more induced abortions and 3.88 (95% CI 2.02-7.50) times higher if the induced abortion was performed at 15 weeks or more gestation relative to no preceding abortion. Spontaneous abortions were not associated with male-biased sex ratios in subsequent births.

Interpretation: High male:female ratios observed among infants born to women who immigrated from India are associated with induced abortions, especially in the second trimester of pregnancy.

Figure 1:

Crude and adjusted odds ratios (ORs) for giving birth to a male live born infant (index birth) among women who had at least 1 recorded spontaneous or induced abortion between the preceding live birth and the index birth compared with women who had no abortion during the same birth interval, from 1993 to 2012, by maternal place of birth and stratified by sex of previous living sibling(s). Values greater than 1.0 indicate an increased likelihood of having a male infant. *Adjusted for maternal age and neighbourhood income quintile. †Also adjusted for maternal education and marital status at arrival, and duration of residence in Canada. CI = confidence interval, FF = both prior living siblings were female, MM = both prior living siblings were male.

Figure 2:

Crude and adjusted odds ratios (ORs) for giving birth to a male live born infant (index birth) among women who had 1, 2 or more induced abortions between the preceding live birth and the index birth, from 1993 to 2012, compared with women who had no abortion during the same birth interval, by maternal place of birth and stratified by sex of previous living sibling(s). Values greater than 1.0 indicate an increased likelihood of having a male infant. *Adjusted for maternal age and neighbourhood income quintile. †Also adjusted for maternal education and marital status at arrival, and duration of residence in Canada. CI = confidence interval, FF = both prior living siblings were female, MM = both prior living siblings were male.

Figure 3:

Crude and adjusted odds ratios (ORs) for giving birth to a male live born infant (index birth) among women who had abortions at < 15 wk or ≥ 15 wk gestation between the previous live birth and the index birth compared with women who had no abortion during the same birth interval, from 1993 to 2012, by maternal place of birth and stratified by sex of previous living sibling(s). Values greater than 1.0 indicate an increased likelihood of having a male infant. *Adjusted for maternal age and neighbourhood income quintile. †Also adjusted for maternal education and marital status at arrival, and duration of residence in Canada. ‡Data were suppressed because of small counts (≤ 5). CI = confidence interval, FF = both prior living siblings were female, MM = both prior living siblings were male.