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This is a very common saying that people make about soon-to-be mothers. But is it really true? Can you tell what the gender of a baby will be based on how the mother's belly is shaped?

Article that explains the belly test.

Programmer
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From your source:

Like any unscientific gender test, it's accurate about half the time

50% isn't much, you might as well flip a coin.

Also from your source:

Many things factor into the shape and size of the belly during pregnancy, though gender does not make the list as far as medical science is concerned.

So even your source admits that this is not based on scientific fact. I haven't found any papers actually analyzing this "test", which isn't really surprising, but the BBC has an article explaining why it will not work:

Two variables determine the nature of a pregnant woman’s bump. The first is the size of the baby. It is true that on average baby boys weigh more at birth than baby girls, and so this could make the bump for a boy slightly bigger. But this small difference in weight does not change the shape of the bump.

The second is the position of the foetus in the womb. If it has its back alongside the mother’s front this makes her belly stick right out. If the baby’s back is parallel with the mother’s back the result is that the abdomen looks flatter. And as the position the developing baby adopts is not dependent on its sex, it is a myth that the shape indicates whether it is a boy or a girl. source

There is also an article on WebMD about this myth:

"How you carry simply has to do with the tone of your muscles and the position that the baby is in," Beard says. These factors, along with your body shape and how much weight you gain during pregnancy -- not the baby's gender -- will determine how low or high your belly sits. source

tim
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    "50% isn't much, you might as well flip a coin" - Or have a child, as it were. – Jimmy M. Aug 01 '16 at 18:51
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    I have a really successful new method of determining the gender of a baby. It always returns female and is accurate ~50% of the time. – orlp Aug 01 '16 at 21:20
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    Since gender isn't binary, the odds are actually less than 50/50; https://en.wikipedia.org/wiki/Intersex – Richard Aug 01 '16 at 21:52
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    @Richard And since gender and sex aren't the same thing, any attempt at inferring gender from prenatal anatomy is doomed to fail; https://en.wikipedia.org/wiki/Transgender – ApproachingDarknessFish Aug 01 '16 at 23:36
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    And since "~" means "approximately", both those comments are irrelevant. – Blorgbeard Aug 02 '16 at 01:08
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    Hey! That's [what Hitler'd say!](https://en.wikipedia.org/wiki/Godwin's_law) - Just speeding things up :) – mgarciaisaia Aug 02 '16 at 02:03
  • @orlp I have a really successful method of determining the gender of a baby that's accurate 100% of the time. It's called ultrasound. – EMBLEM Aug 02 '16 at 03:15
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    @EMBLEM, ultrasound wasn't even right about *how many* of me there were until the later tries. I know it's gotten better over the years, but I'd be wary about claiming "100% accuracy". – Mark Aug 02 '16 at 03:19
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    @orlp : Actually, there was a very successful Gypsy fortune-teller woman active some time ago in my area who could predict the gender in the first month of pregnancy very successfully out of Tarot cards or whatever. Her method: saying one thing and writing the opposite into her big book. If she guessed correctly, the parents were satisfied, otherwise if the parents came back to complain, she took her big book, looked up the date and names in front of the parents, at showed them the "correct" entry, claiming that the parents remembered it incorrectly. – vsz Aug 02 '16 at 06:20
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    I also knew a very successful baby gender prophet who would guess your baby's gender, and if he got it wrong you got your money back. Luckily for him he only needed 50% of his initial income to make a small fortune. – Tom Bowen Aug 02 '16 at 10:18
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    Average birth rates are about 1.06 boys to 1 girl, so any method that's less than 51.4% of the time correct is *really* bad. Also what we're really talking about here is the *sex* of the baby, not the *gender*, the latter is rather more complicated to determine. – Voo Aug 02 '16 at 13:23
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    "50% isn't much" -- in fact, it's as bad as it's possible to be! If the test got the correct answer less than half the time, you'd get a better prediction by taking the test and then using the opposite answer. But a test that's only 50/50 can't be improved by this kind of technique. – David Richerby Aug 02 '16 at 13:27
  • I don't know if it's true but I've certainly heard women in Britain discuss it as fact. The pattern of fat distribution is supposedly different. It would seem relatively easy to test. – TheMathemagician Aug 03 '16 at 11:47