The likelihood of a woman having a son or daughter is 50/50, similar to a coin toss, but research suggests some women may be more likely to have children of only one sex. Dr. Siwen Wang from Harvard University questioned whether this phenomenon can be explained by chance alone.
In the US, data collected from over 58,000 mothers showed that while the ratio of boys to girls at birth is roughly 50:50, there was a correlation between maternal traits and child sex. The researchers compared eight maternal traits with the sex of the children, including height, body mass index, hair colour, and age when first had a child.
The study found that mothers who were taller, had a higher BMI, or were younger at their first menstruation were more likely to have daughters. Those who were older at their first menstruation or had a later sleep schedule were more likely to have sons. While these findings are intriguing, they do not imply a biological mechanism behind the phenomenon, and further research is needed to fully understand this correlation.
The study’s results suggest that while individual characteristics may influence child sex ratios, there is still no single explanation for why some women tend to have more girls than boys in their family.
Source: https://www.newscientist.com/article/2488858-your-chance-of-having-a-boy-or-girl-may-not-be-50-50