Women who give birth to natural twins are probably more reproductively fit to do so, according to a new analysis by University of Utah researchers.
In the group's sample, mothers with natural twins lived longer, were more fertile as they aged and gave birth to more children in shorter periods of time. In line with previous research, the study supports the idea that certain women are more likely to have natural twins, especially if environmental factors are favorable.
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Throughout human evolution, our species has generally favored single births since bearing multiples increases the risk of mortality of mothers and babies in the short-term. In most cases, natural twinning results in fraternal twins derived from two separate eggs. Identical twinning, when one fertilized egg divides into two separate eggs, occurs less often.
Data were pulled from the Utah Population Database and included women born between 1807 and 1899 who reached at least 50 years of age.
Of the sample, 4,600 mothers bore twins while some 54,100 mothers did not. Researchers used this rich dataset to gain a glimpse of natural twinning before the creation of birth control or infertility treatments such as in vitro fertilization. The downside is the study's focus on one population in history, which might have less detail than what's found in medical records today.
It's already known that older women are more likely to conceive natural twins, but what drives this phenomenon is still unknown.
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Some researchers support the insurance ovulation hypothesis, in which women produce more than one egg to increase the chances of becoming pregnant. But when both are fertilized, it's actually a disadvantage for the mother and her children.
Another hypothesis favors the idea that twinning creates greater health risks for mothers. Although it's one piece of the bigger picture,the Utah study provides evidence against both hypotheses.
Instead, it suggests twinning simply occurs among women who are reproductively healthier and can carry more than one child. It's unclear whether the findings can be applied to women who give birth to more than two children at once. Overall, scientists do not think that twinning is healthy, but rather healthier women are more likely to bear twins and live longer after menopause to begin with.
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