What proportion of babies are males? (a series, part 1)

Johnna Wilford tagged me last week in an interesting IG story wondering whether more baby boys are being born right now and wanted to know if there were any studies on this.

And, there are! I’ll highlight just a few in this series on the proportion of babies that are born male.

Let’s start investigating this question by getting a sense of how this usually works.

Some prior research set forth an expectation that more males would be conceived than females because of the timing of hormones during ovulation. But, in 2015, a new study had some pretty surprising findings. The research team found that, at conception, proportions of males are even to the proportion of females (the article does not note whether any intersex zygotes were a part of the study). So hormone timing and ovulation don’t seem to mean more boys; at least in this sample, it’s 50/50. (There are some tricky bits to how they went about this study methodologically, and if you want to chat about that more, I’d love to have that conversation in the comments section!)

But more boys are born anyway.

*Heads up: the rest of this post is going to talk about loss, so if that’s not something you want to hear about right now, go ahead and skip the rest of this post.*

Before we get into how, let me give you a quick reminder of some of the terms in human development. Development starts with a zygote (a fertilized egg), then a blastocyst (a zygote that has developed distinct systems for skin/nerves, digestion/respiration, and muscles/skeleton), an embryo (a blastocyst that has implanted, typically in a uterus), then a fetus (an embryo with differentiated cells). These terms are basically denotations of age and stage, like newborn, infant, and toddler. You can learn more about those stages here.

So in the 2015 study’s dataset, we see that pregnancies start, with zygotes, with equal proportions of males and females. However, more chromosomal abnormality occur for males, so very early miscarriages are more likely for male zygotes, blastocysts, and embryos.

Then, after a few weeks of pregnancy, the sex ratio in loss changes; miscarriage – and abortion – are more likely for female embryos and fetuses, until the third trimester. During the third trimester, a stillbirth is slightly more likely for male fetuses than for female.

In the end, this means that while there are about as many male as female zygotes conceived and implanted, there are more losses of female embryos and fetuses. This would mean that we would expect more male newborns than female newborns.

One caveat to this is that this is how things *usually* work. We’ve also seen patterns where, while and shortly after a country is at war, the proportion of male newborns is higher. Similar effects have been linked to earthquakes and terrorism, too.

If you’re curious why that might be, tune in next week for an explanation of the phenomenon of the “wartime sex ratio.”