Have all girls or all boys? Study suggests the odds aren’t 50/50

Couples who’ve been surprised by a string of baby boys or baby girls could be forgiven for wondering whether the odds in the offspring lottery are more than just pure chance. A new study by Harvard University researchers that examines the birthing records of 58,007 women suggests their hunch might be correct.

The authors of the study, published in the journal Science Advances last week, found that instead of straight 50/50 odds between boys and girls with each birth, the offspring in families with at least three children tend to follow what scientists call a “weighted coin toss,” indicating that each family may have a distinct tendency toward girls or boys.

“We’re seeing a lot of us having only boys or girls,” says Siwen Wang, a doctoral student at the Harvard T.H. Chan School of Public Health and the lead author in the study. That led her and colleagues to wonder “whether it’s simply by chance, or is there any underlying biology explaining these sex clustering within families?”

They analyzed data from the second and third iteration of the Nurses’ Health Study, which tracked just over 146,000 pregnancies across the U.S. from 1956 to 2015.

They found that if the first two children born in a family are girls, the chances of the third child also being a girl are higher than for a boy — and this pattern continues with subsequent children. The same holds true if the first two are boys: The likelihood of having another boy increases, and the trend persists through later births.

“Notably, in families with three boys (MMM), the probability of having another boy was 61%; in families with three girls (FFF), the probability of having another girl was 58%,” the study authors wrote.

Looking for a possible explanation, “one of the things that stands out is age at first birth — that’s associated with a moderately higher chance of having only boys or girls,” Wang says. “If you’re starting your family younger than 23 … [there is] around a 40% chance of having a same-sex family,” she says. If the woman begins having children after age 28, it’s around 50% — a relatively small but statistically significant difference.

The researchers excluded families with only one child and pregnancies ending in miscarriage, stillbirth or twins. Women with a history of infertility treatments were also left out of the study. To avoid bias from parents who stopped having children after reaching a preferred sex ratio, the researchers did not count each woman’s final birth in the analysis.

The researchers speculate that in older women, shorter menstrual cycles could factor into the results and that changes in vaginal acidity might favor the survival of sperm carrying either the Y (boys) or X (girls) chromosome. However, they note that because maternal and paternal ages are often correlated, the father’s age might also play a role. Since the study included only women, the authors emphasize that the exact mechanism remains uncertain.

“We don’t have data about the dad,” Wang says. “We don’t have data about his genetics or even very precise characteristics … which is obviously important for sex determination.”

It’s a concern also raised by Mark Gerstein, a professor at Yale University who wasn’t involved in the research. Gerstein specializes in applying computational approaches to molecular biology. “Given the disparity observed for mothers in the study, one could easily envision an equally impactful relationship with either the genetics or age of the father,” he says. “This seems like a promising area for future research to explore.”

Ralph Catalano, an emeritus professor of public health at the University of California, Berkeley, who also wasn’t involved in the research, thinks that the study’s narrow focus on sex at birth may not show the full picture. “Fewer than 50% of conceptions reach birth. More than 50% are spontaneously aborted [miscarried], and that’s not a random draw,” he says.

Due to immunological and developmental sensitivity, females are more likely to be spontaneously aborted early in pregnancy, while males are more vulnerable to spontaneous abortion later in pregnancy because they are physically frailer, he says. “The sex of infants we see — those who make it to birth — is shaped as much by in utero selection as by sex at conception,” he says.

Another caveat is the composition of the sample. The Nurses’ Health Study that the sample was drawn from is made up of overwhelmingly white women residing in the United States. “Since sex preferences and reproductive behaviors vary across cultures, religions, or countries … the sex ratio distribution pattern observed in our study may not apply to other societies,” the researchers acknowledge.