Not only do mothers of twins live longer, they are generally older than other moms during their last pregnancy, they get pregnant at shorter intervals, and end up having more children than other mothers, even after the extra baby in the twins is taken into account, University of Utah researchers revealed in the journal Proceedings of the Royal Society B.

The authors stress that having twins does not promote longer life, it is the other way round – the healthier a woman is the greater are her chances of bearing twins.

Co-author, Shannen Robson, said:

“Having twins will not make you stronger or healthier, but stronger, healthier women are more likely to have twins naturally. 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.”

Demographer Ken. R. Smith and team gathered data on 58,786 women in Utah born between 1807 and 1899. They married once after 1850, and their husbands were still alive when the women reached 50 years of age. They were all monogamous women.

4,603 of them had twins, while 54,183 had singletons (one at a time).

Robson said of the sample of 4,603 moms of twins:

“(it is) the largest historic natural fertility data set of twin mothers yet published, at least 18 times larger than any previously analyzed historical sample.”

Smith said:

“People are always interested in what affects how long we are going to live. It’s complicated. There are so many factors that contribute to longevity, health and aging.

This study has been able to identify – and it’s a fairly novel result – another important factor that contributes to health and longevity in later years, namely, that women bearing twins appear to be healthier. That innate healthiness is contributing to their ability to have twins, and it is also contributing to their longevity.”

The authors stressed that their study only evaluated women who survived beyond the menopause, and not those who died earlier. Women who have multiple births have a higher risk of complications and even death during childbirth.

Smith added:

“Women having children are fundamentally young and healthy,” he adds. “So the risk of dying in childbirth is quite low. The women who have twins have a somewhat elevated risk of mortality over those [child-bearing] years, but the vast majority of those women reach age 50, and we’re able to observe that they have healthier lives.”

The authors explain that a woman’s health during her childbearing age is the main factor influencing her chances of having twins. Genetics also play a role – a woman whose mother or grandmother had twins is more likely to do so herself.

The superior health of women who have twins is innate, Smith explains, rather than a result of leading a healthy lifestyle – it is in their biological makeup.

The population evaluated in this study was ideal for the researchers’ aims, which was to look at the effect of natural births of twins. During that period there were no available infertility treatments or modern options for birth control.

Smith said:

“We’re saying that women who twin naturally have something that makes them healthier. We are able to see that in these ancestral women because they had many children and had no fertility treatments. They have left a legacy through their descendants who may all share this desirable trait of being healthier.

(For women today) who have access to infertility treatment and who have twins – which isn’t uncommon – we simply don’t know how that will affect their health. We’re not encouraging women to actively seek having twins so they can live longer. It’s not a conclusion we can draw.”

In this study the researchers separated the females born before 1870 and between 1870 and 1899. In 1870 the modern urbanization of Utah began and women were under less pressure to start having children straight away. In both groups, mothers of twins had better health than other mothers.

There were not enough mothers in the study who had triplets or other multiple births, so they were not studied, the authors added. The database did not distinguish from identical twins and fraternal twins.

Below are some highlighted findings from the study:

  • Moms of twins survived for longer after the menopause. After the age of 50 mothers of twins were 7.6% less likely to die each year compared to mothers of singletons.
  • Mothers of twins ended up having more children than other mothers, even after taking into account that having twins would up the numbers anyway. Robson said “By having twins, you are having one more child by definition, but they exceed the definition and thus had more single births as well.”
  • Moms of twins had further babies at shorter intervals, compared to mothers of singletons. The authors write that this is an indication of better physical health of the mother.
  • Mothers of twins’ reproductive spans were longer than other mothers. The period between their first and last birth was longer.
  • Mothers of twins were generally older than mothers of singletons during their last pregnancy.

In an abstract in the journal, the authors wrote:

“We conclude that bearing twins is more likely for those with a robust phenotype and is a useful index of maternal heterogeneity.”

“Twinning in humans: maternal heterogeneity in reproduction and survival”
Shannen L. Robson1and Ken R. Smith
Proceedings of the Royal Society B May 11, 2011, doi: 10.1098/rspb.2011.0573

Written by Christian Nordqvist