Why Some Families Have Only Sons or Only Daughters: "How Old Was Your Mother When She Had You?"

Scientific Evidence Found for Repeated Gender Patterns
Mother's Age and Reproductive Environment Identified as Key Variables

A new study has challenged the common belief that a child's gender is determined randomly with a 50:50 probability. In fact, the analysis suggests that in some families, the likelihood of having children of the same gender repeatedly may be higher.

A study has found that in some families, the probability of having children of the same gender repeatedly may be higher (photo used for illustrative purposes and unrelated to the article). Pixabay

A research team from the Harvard T.H. Chan School of Public Health in the United States analyzed data tracking about 58,000 women and more than 140,000 cases of pregnancy and childbirth from 1956 to 2015. This is known as the "Nurses' Health Study."

The survey found that in about one-third of all families, all children were of the same gender. Notably, there were numerous cases where three or more children were all sons or all daughters. The research team explained that these figures are difficult to account for with a simple 50:50 probability.

This tendency was found to be closely related to the mother's age at childbirth. According to the study, women who had their first child after the age of 28 were slightly more likely to have subsequent children of the same gender.

It is suggested that biological changes may have caused this difference. The researchers believe that as a woman's reproductive environment changes with age, it can affect sperm survival rates. For example, as women age, the vaginal environment can become more acidic, which favors X chromosome sperm and may increase the likelihood of having daughters. Conversely, if the ovulation cycle shortens and the composition of cervical mucus changes, it may create an environment more favorable to Y chromosome sperm, raising the chances of having sons.

Since the strength of these factors varies for each individual, the results can also differ from family to family.

Additionally, the researchers mentioned genetic factors as another possible variable. In some families, the tendency to have children of a particular gender may be inherited genetically. However, the specific role of the related genes has not yet been identified.

The research team emphasized that other factors, such as the father's age, the parents' health status, and lifestyle habits, may also influence gender determination, highlighting the need for further studies that include these elements.

The results of this study were published in the scientific journal Science Advances. The researchers stated, "Gender determination may not be entirely random," but added, "However, the deviation is extremely subtle, and it remains difficult to predict at the level of individual families."