A baby’s sex at birth might not be the equal coin toss that we’ve always believed it to be. A new study published by the Harvard TH Chan School of Public Health, titled, “Is sex at birth a biological coin toss? Insights from a longitudinal and GWAS analysis” challenges this long-held assumption. The study was published on July 18 in the journal Science Advances, and it found that several factors, including a mother's age, genetics, and the sex of older siblings—may influence whether she’s more likely to have a boy or a girl.Patterns Behind the Birth of Boys and Girls: How Was The Study Conducted?The research looked at more than 146,000 pregnancies involving over 58,000 U.S. nurses from the NIH-funded Nurses’ Health Study, covering the years 1956 to 2015. What they found was eye-opening: families with three or more children were more likely to have either all boys or all girls than if the sex of a child was purely random.“If you’ve had two or three girls and you’re trying for a boy, your odds aren’t 50-50 anymore,” explained Jorge Chavarro, professor of nutrition and epidemiology and the study’s senior author, in a quote to The Washington Post. “You’re more likely to have another girl.”The Role of Maternal Age and GeneticsOne of the biggest surprises was the link between maternal age and the likelihood of giving birth to all children of the same sex. Women who started having children after age 28 had a slightly higher chance of giving birth to only boys or only girls.Researchers believe this could be due to biological changes that come with age, such as shifts in hormone levels or physical conditions that may affect which sperm survive and fertilize the egg.Additionally, the study identified two specific genes that appear to be linked with the likelihood of having children of only one sex. While scientists don’t yet know how these genes affect birth sex, their discovery opens the door for further research.Chavarro noted that future studies should explore whether lifestyle choices, diet, or even exposure to certain environmental chemicals might also play a role.Are Families Subconsciously Seeking “Gender Balance”?The study also touches on social patterns. In families with two children, there was a higher chance of one boy and one girl. Researchers believe this may be because couples tend to stop having children once they have one of each sex, possibly due to a preference for a “balanced” family.This idea is supported by another paper published in the Journal of Behavioral and Experimental Economics, which jokingly refers to parents who have three boys or three girls as those who “lose the birth lottery.”When Expectations Don’t Match RealityThe findings also bring up the topic of “gender disappointment”, a real emotional experience for some parents when the sex of their baby isn’t what they hoped for. A 2023 paper in the BJPsych Bulletin found that in many Western families, this disappointment often stems from a desire to experience raising both sons and daughters.In everyday life, these expectations can be seen in subtle remarks like “Four boys? Poor you,” or “Still trying for a girl?”, comments that reflect how deeply society views gender roles and balance, reports CBC.What It Means for Expecting ParentsWhile the science behind birth sex is clearly more complex than a simple 50-50 chance, experts agree it’s not something to stress over. Rather, it’s a reminder that biology, age, genes, and personal choices all come together in ways we’re only beginning to understand.