A federal study based on seven years of births in Utah concludes that mothers should wait at least 18 months after giving birth to get pregnant again and suggests that spacing babies 2 1/2 years apart is best for their health.
The study by the Centers for Disease Control and Prevention concluded that the wait between birth and pregnancy is best at 18 to 23 months.Many American mothers already space their children a couple of years apart, so they won't have two children in diapers and the youngsters will be close enough in age to play together as they grow up.
The average interval between first and second births is about 2 1/2 years, according to the Alan Guttmacher Institute, a reproductive health organization in New York.
"Somehow the body knows that this interval is good for the health of the infant," said Dr. Bao-Ping Zhu, who directed the study published in today's New England Journal of Medicine.
The study was based on 173,205 births in Utah from 1989 to 1996.
Researchers found that while having babies too close together can be bad for the infants' health, having them too far apart may be even worse. Both situations raise the risk that the new baby will be premature or small, which can cause long-term health problems, even death.
Compared with babies born after the ideal interval, those whose moms became pregnant again within six months had a 30 percent to 40 percent greater chance of producing premature or undersize babies. Those who waited 10 years for another child were twice as likely to have an unusually small baby and 50 percent more likely to deliver prematurely.
Telling mothers about this could help reduce health complications in babies, said Dr. Robert Knuppel, chairman of obstetrics at Robert Wood Johnson Medical School in New Brunswick, N.J.
Dozens of previous studies have linked short intervals with a higher risk of small and premature infants, but none determined the best interval. The few studies on long-interval births were less consistent in their findings.
Zhu said babies conceived too soon probably have problems because the mother is recovering from vitamin depletion, blood loss and reproductive system damage from the prior birth -- all while stressed by having to care for a newborn.
He theorized that the reason getting pregnant after a long interval is risky is that the body becomes primed for birth during the earlier pregnancy, with the uterus enlarging and blood flow to the womb increasing. Those benefits decline over time.
Knuppel noted that 90 percent of the Utah women were white, so the results may not apply to minority mothers or those with high-risk pregnancies. Zhu agrees and is conducting a parallel study in Michigan.
The researchers took into account 16 factors that could affect outcomes, such as smoking and drinking, prenatal care and the mother's age, race and education.
However, in an accompanying editorial, Dr. Mark A. Klebanoff of the National Institutes of Health cautioned that the researchers might have missed some other factors that could contribute to the risks of conceiving too soon or too long after delivery.
These include such things as whether the mother had chronic medical problems, planned the pregnancy or had a miscarriage or abortion since the last birth.
Zhu noted many Utah women did choose approximately the best interval between pregnancies: 15 percent of infants in the study were conceived after an 18- to 23-month lag and 43 percent after a lag of 12 to 29 months.
The American College of Obstetricians and Gynecologists says that waiting 18 months to two years between births is best.