There’s this funny phenomenon in western society. Whenever people find out you’re pregnant, they suddenly feel the need to ask intrusive questions, share unsolicited advice and personal stories, and make confident statements about the outcome of your pregnancy. And don’t get me started with the belly touching!

What makes complete strangers so bold in telling you you’re having a girl, or that baby is going to be a looker, is a culture of old wives tales that have been passed down the generations, claiming to predict all kinds of things about your newborn, on the basis of how you look as a mother.

They say that unborn baby girls steal their mother’s beauty. Which is basically like saying you’re not looking your best. And let’s be honest, to a pregnant person that’s pretty rude!

They say that, if your bump is sitting low then you’re having a boy.

And they say that wearing high heels can make your baby cross eyed! Even though crossed eyes are common in newborns, I don’t think you have to be a scientist like me to question how on earth that one would work.

In fact, pretty much all of these old wives tales have absolutely no basis in fact. For every baby that fits the trend you’ll find at least one more that doesn’t. People are just really good at seeking confirmation for their favourite theories, and conveniently ignoring the times the facts don’t support them!

But there are exceptions. One or two of these folk tales actually do end up accurately predicting babies’ characteristics, even though the people who repeat them might not have known why.

A Burning Feeling

When I was pregnant, I had terrible heartburn. Like, I thought I knew what heartburn was before, and I always had a ready supply of Rennie in the cupboard. Thankfully, ‘heartburn’ describes the sensation, and is nothing like what’s actually happening inside your body. Although it can feel like your heart is on fire, it’s actually the nerves in your lower oesophagus complaining that they’re being attached by corrosive stomach acid escaping up from the stomach where it usually belongs. So yeah, I suffered from heartburn because I had a tendency to eat big, rich meals late at night, then lie down and let all that stomach acid leak out.

But when I was pregnant, it stepped up another notch, despite my eating habits not really changing. It got so bad that Rennie wouldn’t really touch it any more, and I turned to these lovely big bottles of antacid that I could chug every time I got up to pee in the night (and that was a lot!).

Whenever I complained of my heartburn to friends, midwives, doctors, or even nosy strangers, I kept hearing the same thing back. “Ooh”, they said, “that means your baby’s going to come out with a full head of hair!”

Now, scientist that I am, and knowing of the ridiculousness of many of these old wives tales, I discounted it as folk fable. But lo and behold, when science baby was born, he came out with a full head of hair! And yes, you could call it confirmation bias and label me no better than every gullible mother through history, but it turns out it’s not just me.

A small study (they all tend to be small, so don’t @ me!) asked 64 mothers how bad their heartburn was during pregnancy, and then collected other data about the mother and baby. They found that the degree of heartburn was not related to the age of the mother, or the sex or size of the baby. But it was related to the amount of hair baby was born with. In this case, the two really do appear to be correlated!

Hair to Heart

Identifying a correlation between heartburn and hair is one thing, but the key to cementing the relationship is to find out the cause. People confuse correlation and causation all the time. So hairy babies and heartburn in mothers are linked. But is the hair causing the heartburn? Or the heartburn causing the hair, or is something else going on?

Baby’s hair grows on baby’s head, which until the moment of birth is inside the amniotic sac inside the womb, so is about as isolated from the mother’s stomach as it’s possible to be while still being inside her. So it’s pretty inconceivable that it’s the hair that is causing the heartburn directly, even if there was some way for it to do so. Neither is it likely that the heartburn itself causes the hair, because something must be causing the heartburn in the first place!

So we’re left with option that there’s something else involved. In the past, heightened heartburn in pregnant mothers has been thought to be caused by baby growing, taking up space in mother’s abdomen, and squashing the stomach so that it’s more likely that stomach acid will escape out the top. But there are two problems with this. For one, it doesn’t explain the correlation to hairy babies. Hair might be a shocking addition to your newborn, but it hardly takes up a lot of extra space in the womb, so it’s not like a hairy baby will squash a stomach any more than a bald one.

But also, women who report heartburn in pregnancy, me included, start to do so on average when the foetus is about the size of a peanut, long before it has grown large enough to have any significant effect on their stomachs.

So, instead of physical changes, it’s actually likely that both heartburn and hairy babies are caused by another factor altogether – hormones.

Chemical Mothers

Anyone who has ever been pregnant, or been within a 100 yard radius of a pregnant person will know that growing a baby is achieved by a wild torrent of hormones in the mother’s body. These chemical messengers are normally responsible for everything from our mood, to our menstrual cycle, and during pregnancy (and for up to two years postpartum) they go into overdrive, helping to coordinate all of the crazy changes to body and mind necessary for bringing a new life into the world.

In particular, the feminine hormones oestrogen and progesterone are at heightened levels thoughout a pregnancy. They coordinate the orchestra of other hormones, trigger development of baby’s hormones, and help to maintain the uterus in tip top condition.

But that’s not all they do. Oestrogen and progesterone have been shown to have a direct effect on the growth of hair. That’s the reason why a pregnant mom’s hair grows so thick and luscious while they’re pregnant (and also the reason it all falls out once the post-partum body dumps all it’s unnecessary hormones!)

But here’s the thing: these pregnancy hormones can also pass through the placenta and into baby’s blood stream, where they have the same effect of stimulating hair growth. So more oestrogen and progesterone in mom’s body means more in baby’s body, and more hair growth for both!

Sphincter Sister

Heightened levels of pregnancy hormones can explain why you might get a hairy baby, but how does it relate to heartburn?

As I mentioned above, the burning sensation inside your ribcage is caused by stomach acid leaking out of your stomach and affecting the sensitive tissue of the oesophagus. The reason it doesn’t do this all the time is a ring of tight muscle called the lower oesophageal sphincter. You might have heard of sphincters before in the context of the anus or anal sphincter, and lovely though the image is, that’s exactly what you’ve got at the top of the stomach as well. Normally, the lower oesophageal sphincter squeezes just tight enough to let food down through it, but unless your stomach is super full, it doesn’t typically let the stomach acid slosh back up.

That is, until oestrogen and progesterone get involved. Because as well as doing all the pregnancy coordinating stuff, and stimulating hair growth, these hormones also have the effect of weakening sphincters throughout the body. Under the effect of oestrogen and progesterone, these rings of muscles aren’t able to squeeze as tightly as they usually do. And with a looser grip on the bag of stomach contents and juices, the lower oesophageal sphincter is less able to stop burning acid from breaching the oesophagus.

So, more oestrogen and progesterone in mom’s body means less control over her sphincters, more leakage of stomach acid and more heartburn.

Will I have a hairy baby?

To summarise, the pregnancy hormones oestrogen and progesterone are responsible for a lot of strange stuff in the body. As well as coordinating the important stuff of growing a baby, they also make mom’s and baby’s hair grow thick and luscious, and they loosen a bunch of sphincters around the body, including the one at the top of the stomach which holds the stomach acid in. So it’s likely that if you’re having a lot of heartburn, your baby will be growing a lot of hair as well. The more of these pregnancy hormones, the more heartburn and the more hair.

The thing is, scientists still aren’t sure why some mothers make more or less progesterone and oestrogen during pregnancy. We all have enough to grow a baby, but some have an extra supply that contribute to that hair growth. And there’s no way of knowing if your body is one that’s good at growing wookies.

At the end of the day, a lot of it is relative. The heartburn you experience might be bad for you, but it’s hard to make subjective comparisons with other expectant moms. Really, you won’t know if you’re having a hairy baby until they make their grand entrance!

Either way, whether your newborn is as hairy as the Beast or as bald as Professor X, it doesn’t really matter. Over their first few months, baby’s hair is likely to fall out, and may even change colour and texture as their own hormones take over.

Read more

• https://pubmed.ncbi.nlm.nih.gov/17150070/