Mothers born in India but living in Canada are more likely to have male babies for their second and third births compared with women in Canada, a new study has found.
Researchers from St. Michael's Hospital and the University of Toronto conducted the study.
"Our findings raise questions about why there are more male liveborns than female liveborns among Indian couples who have had two or more previous babies," said Dr. Joel Ray, St. Michael's Hospital and University of Toronto.
For the study, the researchers looked at all singleton live births in Ontario - 766 688 births - between 2002 and 2007.
They used the mother's country or region of birth to categorize each birth, with 486 599 from Canada, Europe (58 505), India (31 978), China (23 818), rest of East Asia (18 971), Pakistan (18 018), South Korea (3663) and others.
They calculated male:female live births by country or region and the number of previous deliveries the mother had at the current birth (0, 1, 2, 3 or more). They also looked at the country of birth for fathers.
For most women, the male:female ratio was 1.05, and the ratio did not change with increasing numbers of previous births.
However, women from India and South Korea who had previous children were significantly more likely to give birth to males. For Indian-born women with more than one prior child, the male:female ratios were even more pronounced.
Previous research conducted in India and a study looking at Canadian census data for South Asian and East Asian immigrants show similar patterns of higher male: female ratios linked to increasing numbers of children, especially when the prior offspring were girls.
"Whether this difference in sex ratios was the result of prenatal sex selection should be determined by direct study of practices of sex-selected preimplantation and pregnancy termination among individuals from various world regions," said the researchers.
"In addition, an analysis of the duration of residence in Canada, access to fertility care, family income and parental preferences would be of value in describing factors that might influence prenatal sex selection," they concluded.
The study was limited to singleton births and could not account for any sex selection in multiple pregnancies from fertility treatments. The researchers also could not determine the sex of older siblings born to the same mother.
The findings were published in CMAJ (Canadian Medical Association Journal).