When It Comes to Pregnancy, Timing Is Everything

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The researchers reviewed data on more than 11 million pregnancies spanning a 40-year period. They found that babies born to women who had an interval of less than six months between pregnancy and conception were 40 percent more likely to be born early than those whose mothers waited 18 to 23 months. Those babies were also 61 percent more likely to be underweight at birth and 26 percent more likely to be small for their gestational age, compared to infants born to mothers who waited 18 to 23 months between pregnancies.

But, the Colombian researchers also found that babies born to mothers who waited longer than 59 months between delivery and the next pregnancy had a 20 percent to 43 percent increased risk of health problems, such as small for gestational age.

Short intervals may increase risk to the babies, because the mother hasn't had enough time to recover nutritionally from one pregnancy to the next one, experts say. It's not clear why long intervals might pose problems, but one possibility is that women who delay another pregnancy for an extended period of time may simply be reaching a stage in life where advancing maternal age is itself becoming a risk factor.

Rachel Royce is a senior epidemiologist at RTI International, a scientific research institute in Research Triangle Park, N.C., who wrote an editorial to accompany the Colombian research. She noted that "the data show that intervals in the range of 20 to 40 months [between births] are associated with the best outcomes."

That works out to about 12 months or longer between delivery and conception of the next baby -- exactly what the Missouri researchers concluded.

"Essentially, clinicians should counsel all women to space pregnancies at least 12 months apart, if at all possible," Royce said.

DeFranco agreed.

More information

To learn about how to get healthy before getting pregnant, visit the March of Dimes.