If you've ever felt sympathy for a mother of twin infants because it looks like double the trouble, a new study offers a refreshing conversation starter: Mothers of twins appear to live longer than mothers of singletons.
The finding flies in the face of previous research that says having more children reduces a woman's life span.
But it's not the twinning itself that leads to a longer life, says University of Utah researcher Ken R. Smith: It's that the act of having twins appears to "reveal something we wouldn't otherwise see," perhaps a trait of robustness.
Prof. Smith, who teaches family and consumer studies and is director of the Utah Population Database at the university, says he and his colleagues are primarily concerned with longevity and disease prevention. They look at fertility very closely, because biologically fertility is not only about giving birth, but also about ensuring one's offspring reach their own reproductive maturity.
"We started the project because we're very interested in what makes people live a long time," Prof. Smith said. "Why does someone make it to 100 or die at 60?"
With about 7 to 8 per cent of mothers giving birth to twins, he wondered what effect the seemingly burdensome experience would have on them. He and his colleagues used the massive Utah Population Database, one of the world's largest computerized genealogies, which collates the records of more than 1.5 million people in the state from the early 1800s to the mid-1970s.
They narrowed it down to a sample of about 60,000 women born before 1899, and compared mothers of twins with mothers of singleton babies on a number of fronts, including mortality rates after age 50.
Mothers of twins died at a rate of 7.5 per cent less each year compared with the other moms. The effects were strongest in women born before 1870. The study calls this a "natural fertility era," before the use of emerging family-planning methods.
Not only did the moms of twins live longer past menopause, they also had more children in total, shorter gaps between births, more years of reproduction and were older when they gave birth for the last time.
"There may be classes of women who have this capacity, that are particularly robust," Prof. Smith says. "There may be some constellation of traits that allows them to perform well, biologically."
Up next to explore is what exactly is going on, what causes the trait, and how biology, genetics and social settings may affect it. "Does this point to people who might hold something we could bottle?"
The study certainly makes intriguing reading for mothers of twins.
Catherine Hewlett recalls returning from her first play date with other twin moms and asking her husband, "Isn't it weird we're all so tall?" says Ms. Hewlett, who runs the non-profit Toronto Parents of Multiple Births Association.
Hearing that there may be a trait lurking within moms of twins, Ms. Hewlett, who has four-year-old twin boys and a toddler daughter, says it sounds plausible. While her boys are the result of in-vitro fertilization, there's also a history of twins in her family.
"Maybe there's something to being a tall, healthy person."
Prof. Smith acknowledges that the rising incidence of twin births associated with assisted reproduction could complicate matters - his population covered only "natural" twinning - but says it's possible that the success rate of IVF and other technologies are linked to this mysterious longevity trait.
If he could find descendants of his Utah samples, he'd like to see if those who sought fertility treatment were more likely to conceive than others, he says.
If it's true, Ms. Hewlett says, it would be a nice thank you for all those long days of juggling two babies on two different schedules.
"It's a nice little bonus in the end."