Having two babies at a time is associated with a longer life, according to a new study. But that's not because doubling up on dirty diapers increases life span; instead, moms of twins are physically stronger in the first place.
One catch: The research, published today (May 10) in the journal Proceedings of the Royal Society B, focused on a "natural fertility" population of women in 1800s Utah, so the results may not apply in today's in vitro fertilization (IVF) world.
However, the findings do suggest that rather than being a reproductive accident that drains mom of energy and nutrients, twins could be an evolutionary adaption in which healthy moms take the chance to pass on double their genes at once.
"We expected the exact opposite," study researcher Shannen Robson of the University of Utah told LiveScience. "We expected that since most humans have one baby at a time, having two would be really burdensome … [Twins] are an identifier of these women who are remarkable, physically exceptional people."
Identical twins, created when one embryo splits into two during development, appear more or less at random. But fraternal twins, who develop from two separate eggs released and fertilized at the same time, show some patterns of both heritability — they run in families — and environmental influence. Not including twins conceived from IVF, twins account for 6 out of every 1,000 births in Asia, 10 to 20 out of every 1,000 births in the U.S. and Europe, and 40 out of every 1,000 births in Africa.
To look at twinning before reproductive technology and reliable birth control, the team used the Utah Population Database, an enormous genealogical record of Utah residents dating back to the early 1800s. From the database, they pulled family records of women who were born between 1807 and 1899 and who lived to be at least 50, so they experienced their full range of reproductive years. They excluded widows and wives in polygamous families to ensure they were comparing similar women. [The History and Future of Birth Control]
The result was a database of 58,786 women, 4,603 of whom had at least one set of twins. The researchers compared the moms of twins with the moms of singletons, looking for differences in life span, number of children, time between pregnancies and length of fertility, all measures of health.
Double the fun
Flying in the face of the assumption that a double pregnancy would sap a woman's strength, the researchers found that moms of twins beat moms of only singletons on every measure. They lived longer, had longer reproductive life spans, needed less time to recover between pregnancies, and had more children overall. The moms of twins born before 1870 had on average 1.9 more children than moms of singletons in their age group, and the moms in the post-1870 group each had 2.3 kids more than their singleton mom counterparts.
Because twins have a higher likelihood of death than singletons, the researchers adjusted that finding by infant mortality, assuming that a mom of twins might have more babies faster after a child died. After that adjustment, moms of twins still came out ahead, having 1.24 to 1.56 more babies than singleton-only moms. That exceeds the "plus one" effect you get from having twins, Robson said.
The results didn't differ as time went by, even though pioneer women in pre-1870s Utah had worse medical care than women born later. It's hard to compare the 1800s data to today, however, Robson said. IVF has increased the number of twins born. And other factors have changed as well: women have fewer pregnancies overall now than 1800s women in Utah, so their overall chances of having a spontaneous twin pregnancy are lower. One 2001 study of women in rural Gambia, however, did find that mothers of twins had better reproductive health than mothers of only singletons. [Read: 5 Myths of Fertility Treatments]
Robson and her colleagues now hope to look at the Utah women's twins, to see how they fared given the fact that twins are more likely to be premature and have health problems. They also hope to take a closer look at the supermoms who birth twins.
"By identifying them, we can then look at other aspects of what it is about them that makes them more healthy, live longer and have babies at a faster rate than everyone else in the population," Robson said.
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Stephanie Pappas is a contributing writer for Live Science, covering topics ranging from geoscience to archaeology to the human brain and behavior. She was previously a senior writer for Live Science but is now a freelancer based in Denver, Colorado, and regularly contributes to Scientific American and The Monitor, the monthly magazine of the American Psychological Association. Stephanie received a bachelor's degree in psychology from the University of South Carolina and a graduate certificate in science communication from the University of California, Santa Cruz.