The findings "are not surprising, given the stresses that caring for (twins) brings," Dr. John Moore, a pediatrician at Virginia Tech-Carilion School of Medicine in Roanoke who was not involved in the study, told Reuters Health.

Twins now make up between 3 and 4 percent of all births in the US. And as twins become more common with the increased use of fertility treatment, those small differences in divorce rates may add up, researchers say.

For the new study, Dr. Anupam Jena of Massachusetts General Hospital in Boston and his colleagues used data from the 1980 census to compare the marital status of parents who had twins as their first-born children versus those that did not - a total of more than 800,000 families.

In total, about 13 percent of mothers with a non-twin eldest child reported being divorced and not living with their child's birth father, compared to about 14 percent of mothers who had twins in their first birth.

Having twins made a bigger difference in parents' chance of divorcing when mothers had lots of kids and less income. Older twins and twin pairs that included at least one girl also drove up divorce rates more than young twins and boy-boy pairs, according to the findings published in Obstetrics & Gynecology.

Dr. Elizabeth Damato, who studies parents of twins at Case Western Reserve University in Cleveland but was not involved in the study, said that stress and lack of sleep may be even worse in parents of twins than in parents with one baby, and that those factors may contribute to higher divorce rates.

Financial factors also likely play a role, Jena said, especially as children get older and there are two sets of clothes to buy and two sets of activities and educations to fund at the same time. With twins, clothes and cribs can't be purchased once and passed from one to the other, as with non-twin siblings.