Twin Mothers Aren't More Fertile – Just Luckier

hubie writes:

The prevailing belief by researchers has been that mothers of twins are more fertile than other women. But a new study shows that isn't the case.

Twin births aren't exactly common. In humans, they occur in one to three per cent of all births.

Previous studies of the phenomenon have concluded that mothers of twins are more fertile than other women. This is because on average they give birth more often than other mothers. They have been called "supermothers" and are considered more robust and in better health than mothers of single babies.

Now an international research group has found that twin mothers are not actually more fertile than other women.

"On the contrary, when a woman gives birth several times, the chances increase that at least one of these births will be a twin birth. Twin mothers aren't supermothers, but have been given more chances," says Gine Roll Skjaervo, a senior engineer and human behavioural biologist at NTNU's Department of Biology.

[...] "Previous studies are problematic because they can't tell us whether mothers with twins give birth more often because they're especially fertile, or because giving birth more often increases the chance that one of these births will be to twins," said lead author Alexandre Courtiol at the Leibniz Institute for Zoo and Wildlife Research in Germany.

The new results show that women who give birth to twins are not unusually fertile. The previous research mixed up cause and effect.

"If a mother gives birth more often, it's more likely that one of these births will be to twins - just like you're more likely to win if you buy more lottery tickets, or to be in a car accident if you drive a lot," says Ian Rickard, a lead author from Durham University in the UK.

When taking into account this lottery effect, the researchers find that mothers of twins actually give birth less often than others, not more often. This new finding is in stark contrast to previous ones.

Journal Reference:
Rickard, Vullioud, Rousset, et al., Mothers with higher twinning propensity had lower fertility in pre-industrial Europe, Nat Comm, 2022. DOI: 10.1038/s41467-022-30366-9

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