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.
