A U.S. study finds that a baby’s sex may not be up to chance.

• A new study published in Science Advances found that a baby’s sex may not be totally up to chance.
• By examining more than 58,000 U.S. women with two or more live births between 1956 and 2015, scientists found that families may be more likely to produce one sex over another.
• Researchers also found maternal age is one factor that may play a role in a baby’s sex. 

It’s often said that a baby’s sex is as random as a 50/50 coin toss, but scientists are now saying that’s unlikely. 

A new study, published in Science Advances, examined a population data set of more than 58,000 U.S. women with two or more single live births between 1956 and 2015 and found that individual families may tend to produce more offspring of one sex than the other.1 

It’s as if each family has their own weighted coin when it comes to their baby’s sex, explains Siwen Wang, a doctoral student at Harvard T.H. Chan School of Public Health and the lead author of the study. 

In the study, the probability of a family of three girls having another girl was 58%, while the likelihood of having another boy after three boys was slightly higher at 61%. So, while the “coin” may be biased toward one sex, it’s still possible to have either following a consecutive string of female or male births. 

But that wasn’t the only interesting thing scientists found. 

How Age May Influence Sex 

One pattern that emerged was around maternal age. According to the study, those over 28 years old may have around a 10% higher chance of producing all boys or girls compared to those under 23 years old.

Fertility expert Alex Robles, MD, of Columbia University Fertility Center, says this hints at how the gradual hormone level changes, egg quality, and uterine environment as people age may possibly influence “early embryo development and viability.”

However, that isn’t to say that an older person will only have one sex or another. There are many factors at play here. 

“Older maternal age is usually highly correlated with older paternal age,” says Wang. “And then it is possible that paternal age might play a role in how sex and conception is determined, but we just don’t know about that.”

Since this study strictly looked at mothers, Wang hopes future studies will also consider paternal influence. 

Researchers Considered Parental Preferences  

Sometimes parents prefer a certain number of boys or girls and choose to stop having children when they reach that desired outcome. Researchers call this “coupon collecting.” Taking this into consideration, Wang’s team chose to exclude the last child born to every woman participating in the study. 

For example, if a family of two girls tried for a third baby hoping for a boy, the third child was not included in the analysis. Interestingly, though, when the researchers did include the last pregnancy in older women, the sex predictions remained the same. 

What Doesn’t Seem To Influence a Baby’s Sex 

Some folk tales say that attractive parents are more likely to have daughters, while tall, wealthy, or aggressive parents may produce more sons. 

While the purpose of this study wasn’t to refute this anecdote, the researchers did look at whether race, natural hair color, blood type, height, or body mass index (BMI) of a mother at age 18 influenced a baby’s sex. None of them seemed to have any influence on it. 

More likely, Wang says, there are many small factors working together in a complex way to determine a baby’s sex. 

The Key Takeaways for Parents  

Dr. Robles believes this study shows that “you’re not alone, and it’s not just chance. There may be real biological reasons why some families tend to have all boys or all girls. It’s not something you caused or can easily change—but it’s part of the natural variability of human reproduction.”

He also dispels the myth that timing intercourse or eating certain diets will sway a baby’s sex. Biology is much more complex and unpredictable than we assume, rarely working in perfect averages, he shares.

Wang cautions that the study was not intended to predict future births or encourage interventions, but to build on scientific understanding and encourage future research on how sex is determined in utero.

She just hopes parents will focus on having a healthy pregnancy and the joy of bringing a new life into their family.

Source: https://www.parents.com/why-do-some-families-only-have-boys-or-girls-11780893