Parents have always had a lot of reasons to plan carefully how far apart they space their kids. Now there's one more: Get the timing wrong and you can dramatically increase the risk of having a premature or underweight baby (less than 5.5 pounds). That's the conclusion of a massive meta-analysis—a sort of study of studies—in the April 19 issue of the Journal of the American Medical Association.

Around the world, more than four million children die each year in the first four weeks of life and 28% of those deaths are thought to be related to prematurity or low birth weight. Those numbers don't take into consideration the impact preterm birth or small size can have on overall health and mental development of the babies who do survive.

Investigators gathered research papers on childbirth published across a 40-year period from 1966 to 2006—reflecting more than 11 million pregnancies—to determine the role birth spacing played in the babies' size. In general, the study authors found that there was a wide sweet-spot of 18 to 59 months in which timing does not have much effect.

For every month shorter than 18 months, however, there was a 1.9% greater risk of a preterm baby and a 3.3% jump in the likelihood of a low-birthweight baby. For each month beyond 59 months, those same risks increased 0.6% and 0.9%, respectively. None of those figures by themselves is overly troubling. But the months go quickly and if you fall even a bit too far on either side of the divide, the danger grows just as fast.

What it Means: The answer, as with so many other childbirth worries, is family planning. Nothing will entirely eliminate the risk of the unplanned pregnancy—the so-called oops baby. But simple measures can help: good contraception, for example, and awarenes of mom's ovulatory cycles and how those cycles can change over time.

Breastfeeding exclusively for the first six months of the child's life can also make an enormous difference. Not only does breast milk confer all manner of nutritional and immune advantages on a newborn, but it also delays the resumption of the mother's menstrual cycle. While this is not by itself an effective means of birth control, it does make conception a lot harder.

From the Archive:
3/08/1999 The Real Truth About the Female Body