And no, the full moon has nothing to do with the timing of the birth.
But in recent years, a handful of studies have found that some of the most outlandish old ideas concerning babies and pregnancy may have some scientific merit after all.
Here are a few that superstitious grandmothers everywhere can put in the win column.
Researchers aren’t quite sure why this one’s true, but nonetheless: In 2003, a team of doctors analyzed more than 8,000 births at a single hospital in Ireland between 1997 and 2000, excluding women who delivered prematurely or needed to induce labor. When they crunched all the numbers, the difference between the sexes was small but noticeable.
On average, labor for boy births lasted a little over six hours, while girl births took a little under six. Women delivering boys were also more likely to run into complications during delivery, requiring C-sections 6 percent of the time (compared to 4 percent for girls) and forceps in 8 percent of cases (as opposed to 6 percent).
Overall, 29 percent of boy deliveries ended up requiring some sort of extra intervention; for girls, it was 24 percent.
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One possible reason for the discrepancy: Boys, on average, weigh three and a half ounces more at birth than girls do. And a small 2003 study found that women carrying boys also consume more calories during pregnancy, suggesting that male babies are a little more demanding even before delivery starts.
To have a girl, stay away from bananas
It’s not just that women carrying boys will eat more; eating more in the lead-up to pregnancy may also make it more likely that the baby is a boy.
In 2008, a study published in the journal Proceedings of the Royal Society Basked 740 women who were pregnant for the first time to describe their diets over the year before they conceived, then divided them into three groups based on calorie intake.
The women in the top third of the study, calorie-wise, had boys 56 percent of the time; among the third that consumed the fewest calories, 55 percent had girls.
‘Just relax’ has always been bad advice for getting pregnant.
Here’s where the old line about bananas — that eating them before conception will up your chances of having a boy — comes in. The study authors found that sheer calorie intake wasn’t the only thing that made a difference; specific nutrients did, too.
Eating plenty of potassium (which bananas are chock-full of) was linked to boys, as were diets high in calcium and sodium.
Though as the Guardian noted, overhauling your diet to influence the sex of a future kid can be a risky move, health-wise, spurring people to overload on some nutrients and to ignore others — a safer idea may be to focus on eating healthily in general, and leaving the rest up to chance.
Heartburn is a harbinger of a hairy baby
Here’s a horrifying fact about babies: Sometime in the second trimester of pregnancy, they develop a coat of fine hair all over their bodies, called lanugo — and sometime in the third trimester, they shed it and eat it. A baby’s first poop, in fact, is the digested remains of their own body hair.
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The hair on their heads, though, is another story. While all babies have lanugo in the womb, not all babies are born with anything up top — and mothers looking to know ahead of time whether their kid will be fuzzy-headed or bald may be able to use their own heartburn as a clue.
In 2006, for a study in the journal Birth, a team of researchers from Johns Hopkins University followed 64 women throughout their pregnancies, 28 of whom reported moderate or severe heartburn.
Within that group, 23 women ended up having babies of average or above-average hairiness. On the opposite end of the spectrum, of the 12 women who reported no heartburn at all, ten of them gave birth to kids who were less hairy than the average newborn.
But here’s a perfect example of correlation not equaling causation. The heartburn wasn’t a result of the hair, or vice versa. Rather, the study authors argued, both were caused by a third factor: pregnancy hormones.
Specifically, the same hormones that contribute to hair growth in the womb have also been shown to relax the muscles that keep stomach acid contained in the stomach.
So you can technically blame the baby for that fiery pain in your chest, but the link isn’t quite as direct as the old wives’ tale implies.