By Kathleen Doheny
EmpowHer's HealthDay Reporter

THURSDAY, April 3 (HeathDay News) -- First, there's the initial baby question: When is the right time to have your first child?

Turns out, that's just the beginning. Equally important is figuring out when to have the next baby -- if you decide to have more than one.

The proper timing of pregnancies, experts say, can decrease your risk of having a baby born premature and with a host of health problems.

Intervals that are too brief -- as well as those that are too long -- aren't desirable, research suggests.

In one of the latest studies, researchers from Washington University in St. Louis evaluated more than 156,000 women from Missouri who had two births from 1989 to 1997. The researchers looked at the intervals between pregnancies and the outcomes for those pregnancies. The study, led by Dr. Emily DeFranco, a clinical fellow in maternal-fetal medicine and a staff member at Washington University's Center for Preterm Birth Research, was published recently in the American Journal of Obstetrics and Gynecology.

Among the findings, according to DeFranco:

* Intervals of less than six months from delivery to conception of the next baby increased the risk of preterm birth (less than 35 weeks) by 41 percent.
* Intervals of six to 12 months increased preterm birth risk by 14 percent.
* Intervals of 12 to 18 months carried no significant increased risk of preterm birth.

"Wait a minimum of 12 months before becoming pregnant again," DeFranco advised. That's to say, let 12 months or more go by after delivery before you start trying again to become pregnant. That advice is especially crucial for women who have already had a preterm birth, which raises the preterm risk in subsequent pregnancies, she said.

In another report, published in April 2006 in the Journal of the American Medical Association, researchers from Bogota, Colombia, reviewed the results of 22 published studies, coming to conclusions similar to those of the Missouri researchers. But the Colombian researchers also found hazards with intervals that were too lengthy.

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.