Is it a boy or a girl? That’s the most common question I hear during ultrasounds. Many couples want to know before the 20-week ultrasound. And there are plenty of old wives' tales that patients reference when guessing the sex of their baby.
My general response is that it's a 50/50 chance that a woman will have a boy or a girl. But that's not exactly true – there’s actually a slight bias toward male births. The ratio of male to female births, called the sex ratio, is about 105 to 100, according to the World Health Organization (WHO). This means about 51% of deliveries result in a baby boy.
While the sex ratio can be distorted by populations that selectively value male over female births, there could be another explanation. Research suggests the slight natural skew of the sex ratio could be nature’s way of adjusting for higher death rates in males due to injuries, accidents, and war.
For example, in England around 1900, 50.8% of births were boys. Following World Wars I and II, the rate of male births increased to 51.6%. This may not seem like a big difference, but it resulted in 32 more boys than girls born for every thousand births. Similar changes were seen in other European countries as well following these wars.
It seems like sex ratio shifts should be a random phenomenon. But from a medical standpoint, perhaps there’s a genetic explanation to changes in the numbers of boy and girl babies at different times in history.
It seems like sex ratio shifts should be a random phenomenon. But from a medical standpoint, perhaps there’s a genetic explanation to changes in the numbers of boy and girl babies at different times in history.'
It's all about Dad's genes
Let’s return to the ultrasound discussion. When I tell patients there's about a 50/50 chance for either sex, I also tell them the father's genes determine the baby's sex since some of his sperm carries X chromosomes and some carries Y chromosomes.
That’s with the assumption, though, that the man's sperm carries equal numbers of X and Y chromosomes. A man's X and a woman's X combine to become a girl, and a man's Y combines with a woman's X to become a boy. But if the sperm don’t have equal Xs and Ys, or if other genetic factors are at play, it can affect the sex ratio.
Researchers in England set out to determine whether this is true. They downloaded family trees from the Genealogy Forum, then eliminated data they felt weren’t accurate – for instance, people reported as having more than two parents or a discrepancy in an individual's sex. This left researchers with 927 family trees that had at least three generations and included over half a million individuals dating back to 1600.
Their findings were telling. In the computer models, when researchers removed men from the population data before they had a chance to start families, there was an increase in the number of male babies born in the next generation. The researchers also found that the sex ratio for families followed the father's side, not the mother's side. For example, if a man had more brothers, his own children were more likely to be male; if he had more sisters, he was more likely to have daughters. This was not found to be the case for women.
According to this study, the explanation might be due to a gene that controls the balance of X- and Y-carrying sperm. Men carrying a gene that leads to their sperm having more Y chromosomes have more sons. During times of war and large casualties of male soldiers, those families are more likely to have more surviving sons. And when those men have children, they, like their fathers, might be more likely to have baby boys. This could account for the temporary increase in the sex ratio for that time period.
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What the sex ratio means for today's pregnancies
In a stable population, there is typically a balance between the likelihood of a man fathering sons vs. daughters. But it’s interesting to think that in times when large numbers of men died or were killed before having children, nature might have found a way to correct the population imbalance.
If you're wondering whether your baby-to-be will be a girl or a boy, look at your partner's siblings for clues. Remember, this type of research is more significant at the population level than at an individual level. But I’m still interested in hearing your family stories during your ultrasound.
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