A million and one old wives' tales predict the sex of an unborn child in early pregnancy.
For example, according to some, if your bump is high, you are having a girl, but if it is low, a boy is on the way.
Similarly, if you crave ice cream every day, you can expect a female. Others believe that sex can be predicted by mixing urine with a tablespoon of Drano - if it turns green, it's a girl, and if blue, a boy.
Another commonly held myth is that women pregnant with girls are more likely to break out in zits.
Unsurprisingly, none of the above are backed up by scientific research. To date, there seems to be no way to predict the sex that a mother will go on to produce.
Stepping away from humankind, much research has been carried out on ways to predict the sex of farm animals' offspring. In cows (and other animals), for instance, the Trivers-Willard hypothesis states that female animals in a better physical condition are more likely to produce male offspring.
Some scientists argue that, in a similar way, human maternal diet before conception might be able to predict fetal sex before conception. Women with the best diets produce more males and, conversely, women who consume less energy are more likely to produce female offspring.
Although this theory is controversial and not universally accepted by any means, it does open up the possibility of predicting the sex of a child who is yet to be conceived - or at least establishing conditions that might increase the likelihood of giving birth to one sex over another.
A large study, published this week in the American Journal of Hypertension, set out to investigate this age-old question with fresh eyes. Could a pattern be found?
Sex and blood pressure
Dr. Ravi Retnakaran - an endocrinologist at Mount Sinai Hospital in Toronto, Canada, and an investigator with the Lunendfeld-Tanenbaum Research Institute - started recruiting for the current study in 2009. In all, 1,411 female participants from Liuyang in China took part in the trial.
The women were assessed for cholesterol, blood pressure, glucose, and triglycerides. These measures were taken at an average of 26.3 weeks before becoming pregnant. In total, the pregnancies resulted in 739 boys and 672 girls.
Analysis showed that mean adjusted systolic blood pressure prior to pregnancy was higher in women who eventually gave birth to a boy.
Once the researchers adjusted for age, smoking, education level, waist circumference, body mass index (BMI), triglycerides, low-density lipoprotein (LDL) cholesterol, high-density lipoprotein (HDL) cholesterol, and glucose, the effect was still significant. Women who delivered a boy had a mean systolic blood pressure of 106.0 millimeters of mercury, compared with 103.3 millimeters of mercury for a girl.
According to Dr. Retnakaran, the finding "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. This novel insight may hold implications for both reproductive planning and our understanding of the fundamental mechanisms underlying the sex ratio in humans."
The results are intriguing, but only further studies will conclusively demonstrate whether this effect is genuine.
Maternal blood pressure before pregnancy and sex of the baby: A prospective preconception cohort study, Ravi Retnakaran et al., American Journal of Hypertension, doi: https://doi.org/10.1093/ajh/hpw165, published online 12 January 2017, abstract.
Oxford University Press USA news release, accessed 12 January 2017 via EurekAlert.
Additional source: Pre-conception energy balance and secondary sex ratio - partial support for the Trivers-Willard hypothesis in dairy cows, J. R. Roche et al., Journal of Dairy Science, doi: http://dx.doi.org/10.3168/jds.S0022-0302(06)72282-2, published online June 2006, abstract.
Additional source: You are what your mother eats: Evidence for maternal preconception diet influencing foetal sex in humans, Fiona Mathews et al., Royal Proceedings of the Royal Society B, doi:10.1098/rspb.2008.0105, published online 22 July 2008.