For centuries, mamas have been trying to predict the sex of their baby by assessing how they look, feel or carry. And then there's food—some mamas might believe their pregnancy cravings can either reveal or sway the result. But as far as accuracy is concerned, the interest is equal to the debate. (Spoiler alert: Absolutely none of these have been conclusively supported by robust scientific evidence.)

However, a more dependable way to predict a baby's sex was recently discovered.


While studying the health of prepregnant women as an indicator of a population's boy-girl ratio, researchers found a connection between a mama's blood pressure in the weeks before conception and the sex of her baby. Higher systolic blood pressure tended to result in boys, while lower blood pressure resulted in girls.

Systolic blood pressure is the top number in the reading that indicates the force your heart exerts on the walls of your arteries each time it beats (diastolic blood pressure is the bottom number that indicates the force in between beats).

In this new study, led by Dr. Ravi Retnakaran of Mount Sinai Hospital in Toronto and published in the American Journal of Hypertension, 1,411 women in China who were planning to get pregnant were examined at about 26 weeks prior to getting pregnant.

The study results revealed that, for the women who went on to become pregnant, the higher their systolic blood pressure was at 26 weeks before pregnancy, the higher their chance was that they would deliver a boy.

On average, the 672 mamas who delivered girls had a systolic blood pressure of 103.3 mm Hg, versus the 106.0 mm Hg of the 739 mamas who delivered boys. The data also revealed that if a mama's systolic blood pressure got as high as 123 mmHg, her chance of having a boy was 1.5 times higher than of having a girl.

According to Dr. Retnakaran, "[This] suggests that a woman's blood pressure before pregnancy is a previously unrecognized factor that is associated with her likelihood of delivering a boy or a girl."

3 ways higher maternal blood pressure before pregnancy has emerged as a strong independent predictor of delivering a boy:

  1. Statistically, the difference in blood pressure was big (almost 3 mm Hg)—both before and after researchers adjusted for age, education, smoking, body mass index, cholesterol, triglycerides and glucose.
  2. As a predictor of a baby's sex, no other maternal characteristic was nearly as significant or consistent as systolic blood pressure before pregnancy.
  3. The difference in systolic blood pressure between future mothers of boys and girls was easily observed before mamas got pregnant—but was not evident during any trimester of their pregnancy.

Keep these limitations in mind:

  • The findings do not indicate that higher systolic blood pressure causes a mama to have a boy, but merely suggests that there is an association.
  • The study was performed in young, healthy Chinese women with normal weight and may not be applicable to other populations.
  • It has not been demonstrated that a mama can increase her chance of delivering a boy by deliberately raising her blood pressure (researchers caution against this).

No one actually knows how blood pressure may affect a baby's sex, but other studies indicate the early process in how the placenta is formed seems to be different, depending on the sex of the baby. Changes in a mama's vascular function are needed in early pregnancy to accommodate the increased blood flow required by the baby and placenta combined, so a mama's blood pressure potentially may be relevant to the early development of the placenta in a sex-specific manner.

Who knew when they set out to understand what underlies the human sex ratio that the researchers would discover a previously unrecognized factor associated with the likelihood of having a boy or girl? Only time can tell what implications this discovery may have on the future of reproductive planning.


Sources:

Brown ZA, Schalekamp-Timmermans S, Tiemeier HW, Hofman A, Jaddoe VW, Steegers EA. Fetal sex-specific differences in human placentation: a prospective cohort study. Placenta. 2014 Jun;35(6):359-64. doi:10.1016/j.placenta.2014.03.014

Retnakaran R, Wen SW, Tan H, Zhou S, Ye C, Shen M, Smith GN, Walker MC. Maternal blood pressure before pregnancy and sex of the baby: a prospective preconception cohort study. American Journal of Hypertension. 2017 Apr 1;30(4):382-8.doi:10.1093/ajh/hpw165]

Rosenfeld CS. Periconceptional influences on offspring sex ratio and placental responses. Reproduction, Fertility and Development, 2011;24(1):45-58. doi:10.1071/RD11906