Twins born to women over 35 had a sharply lower mortality rate than twins born to moms under 25 in the study, which appears in the December issue of Pediatrics. This surprising finding may also show the need for extra education and support for younger mothers of twins, according to the researchers.
"These findings were somewhat unexpected," says the study's lead author, Dawn Misra, an associate professor in the School of Public Health at the University of Michigan in Ann Arbor. "In singleton births, the extremes of age -- the very young and the older mothers -- had the highest mortality," she explains. "But with twins, the older you got, the better off your twins were."
While the researchers didn't investigate the causes of death, Misra says that older mothers probably have a higher education, more money and more resources to help them cope with having two babies at once.
Most of the infant deaths for the younger mothers of twins occurred when the babies were between one and 12 months old. Misra says the leading causes of death in that age group are sudden infant death syndrome and accidental injury. She says prevention strategies are available that might help reduce the number of these deaths.
For the study, Misra and her colleagues gathered data on more than 22 million single births and more than 500,000 twin births that occurred during three different time periods: 1985-86, 1990-91, and 1995-96.
For mothers between 15 and 19 years old, the rate of twin births was 1.5 percent or less during the time of the study, compared to 2 to 3.6 percent for mothers over 40. The rate of having twins rose 67 percent for mothers over 40 during the time of the study.
In single births, the youngest mothers -- those between 15 and 19 -- had the highest infant death rate, 11 per 1,000 babies born, compared to mothers over 40 who had an infant death rate of 8.9 per 1,000 births. The group with the lowest infant mortality was mothers between 30 and 34, with six deaths per 1,000 births.
With twins, the death rate rises dramatically. Mothers under 19 had a mortality rate of 73 for every 1,000 births, while those 20 to 24 had an infant death rate of 47 for every 1,000 births. The rate dropped steadily with every age group in the study, and did so consistently throughout the three different time periods. For mothers over 40, the infant death rate in twins was 20 per every 1,000 live births.
Dr. Mark Werner, an obstetrician and gynecologist at William Beaumont Hospital in Royal Oak, Mich., says that the study results are somewhat surprising, but that they do make sense.
"Twins are fun, but they're also stressful and overwhelming," he says. And, he says, older women may be more motivated to attend their prenatal visits and follow the doctor's instructions. They also may be more financially stable and able to take off from work and get the necessary rest, he adds.
"Rest plays a big part in getting to full-term with twins," he explains. "The longer you're pregnant, the bigger your baby. The bigger your baby, the healthier the baby is."
Misra says these study findings point to the need for more research and may indicate the need for more education and support for younger mothers pregnant with twins.
What To Do
For more information on having twins, read this from the American College of Obstetricians and Gynecologists or this from ePregnancy.com that details the risks of being pregnant with twins.
To learn more about preventing sudden infant death syndrome, go to the American SIDS Institute.