Researchers seek 'optimal' pregnancy interval

Pregnancies spaced less than 18 months or more than 59 months apart carry a higher risk of low birth weight, preterm birth and small size for gestational age.

"This sort of upholds the conventional wisdom that you want to wait between pregnancies, and you want to plan your pregnancies," says Dr. Jennifer Wu, an obstetrician/gynecologist at Lenox Hill Hospital in New York City. "The conventional wisdom is to try to space the births two years apart at least. A family needs to recover physically, emotionally and financially between babies. It needs to be able to devote enough time to each child."

Wu was not involved in the study, which appears in the April 19 issue of The Journal of the American Medical Association. "Having these accumulating studies adds strength to the conclusion," adds Dr. Peter Bernstein, an associate professor of obstetrics and gynecology at Montefiore Medical Center/Albert Einstein College of Medicine in New York City. Bernstein was on a select panel making recommendations to the U.S. Centers for Disease Control and Prevention for guidelines on preconception care coming out this Friday.

Although experts were hesitant to suggest an optimal interval, Wu says that ideally a couple would want to wait 20 to 40 months between pregnancies, with the earliest interval being nine months after the first delivery. Eighteen months is considered optimal by many.

Previous research had suggested that both short and long intervals between pregnancies increased the rates of adverse outcomes, but it wasn't clear if other factors (for example, socioeconomic status or mother's health) also played a role. For this study, researchers at Fundacion Santa Fe de Bogota in Colombia conducted a meta-analysis of studies published between 1966 and 2006. Sixty-seven articles met the criteria for inclusion in the study, representing more than 11 million pregnancies.

The evidence showed that babies born to women who had an interval of less than six months between pregnancies had a 40 percent increased risk of preterm birth, a 61 percent increased risk of low birth weight and a 26 percent increased risk of being small for their gestational age, compared to children of mothers with an interval of 18 to 23 months between pregnancies. Babies born to mothers with pregnancy intervals longer than 59 months had a 20 percent to 43 percent increased risk of these outcomes.

For each month that the pregnancy was shortened from 18 months, the risk for preterm birth, low birth weight and small for gestational age increased by 1.9 percent, 3.3 percent and 1.5 percent, respectively.

For each month that the time between pregnancies was lengthened beyond 59 months, the risk for increased by 0.6 percent, 0.9 percent and 0.8 percent, respectively.

It's not clear why short intervals make for worse outcomes, although several theories have been put forth. One is the maternal nutritional depletion hypothesis, which suggests that the mother doesn't have time to recover from one pregnancy to the next.

Nutritional deficiency in the mother means the child doesn't get enough nutrients either.

It's even less clear why extra-long intervals run into more problems. "It may be related to the fact that women who have long intervals are getting older, and women of advanced maternal age have an increased risk of some of these things," Bernstein says.

The paper is, in a sense, an argument for family planning.

"You don't want to do it too soon," Wu says. "You want to plan a pregnancy."

"I don't know that providers are out there telling their patients at their postpartum visit you really should really try and not have another pregnancy for 18 months," Bernstein adds.

"Health care providers need to start identifying this as a high-risk problem, and they need to counsel patients about planning."

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c.2006 HealthDay News