A Canadian study has discovered a potential new way to predict whether expectant parents are having a boy or a girl and it’s not what you might think.

The research was conducted to find out what factors could help determine the sex of a baby before it’s born. Dr. Ravi Retnakaran from Mount Sinai Hospital in Toronto, and his team gathered together a group of 1,411 women residing in Lluyang, China who were planning to get pregnant within the upcoming year.

The group were then tested for blood pressure, cholesterol, triglycerides, and glucose and all assessments were carried out approximately 26.3 weeks before conception.

After the initial tests, 739 boys and 672 girls were born to the women in the group and the research team discovered blood pressure had a big part to play in which sex the baby became.

The mean systolic blood pressure in women who had boys was 106 mm Hg, compared to 103 mm Hg in those who had a girl. This reading was determined months before conception.

It is thought that in stressful conditions, boys are physically the stronger sex and in evolutionary terms, will more likely survive a stressful pregnancy than a girl so the fetus becomes a boy for survival reasons.

“When a woman becomes pregnant, the sex of a foetus is determined by whether the father’s sperm provides an X or Y chromosome and there is no evidence that this probability varies in humans,” stated Dr Retnakaran. “What is believed to vary is the proportion of male or female fetuses that is lost during pregnancy. This study suggests that either lower blood pressure is indicative of a mother’s physiology that is less conducive to survival of a male foetus or that higher blood pressure before pregnancy is less conducive to survival of a female foetus. This novel insight may hold implications for both reproductive planning and our understanding of the fundamental mechanisms underlying the sex ratio in humans.”

Details of the study were published in the journal American Journal of Hypertension.