Healthier women tend to produce twins

Healthier women have an increased chance of delivering twins, says demographer Ken. R. Smith, senior study author and professor of family and consumer studies, University of Utah.

"The prevailing view is that the burden of childbearing on women is heavier when bearing twins. But we found the opposite: women who naturally bear twins in fact live longer and are actually more fertile," Smith adds, reports the journal Proceedings of the Royal Society B.

The research was based on data for 58,786 non-polygamous Utah women who were born between 1807 and 1899, lived to the age of 50 and married once after 1850 to husbands who were alive when their wives were 50.

Of those, 4,603 were the mothers of twins and 54,183 gave birth only to one baby at a time.

The records came from the Utah Population Database, which is among the world's most comprehensive computerized genealogies and includes vital records of largely Mormon migrants to Utah and their Utah descendants, according to a Utah University statement.
The sample of 4,603 mothers of twins is "the largest historic natural fertility data set of twin mothers yet published, at least 18 times larger than any previously analyzed historical sample", says Shannen Robson, the study co-author and doctoral scholar in anthropology at Utah.

"This study has been able to identify... another important factor that contributes to health and longevity in later years, namely, that women bearing twins appear to be healthier," Smith says.

"That innate healthiness is contributing to their ability to have twins, and it is also contributing to their longevity."

While twinning (birth of twins) sometimes runs in families, previous studies have shown environmental factors are more important - factors such as greater health in the mother or childbearing at later ages, which are more likely to produce twins.