Spacing Births May Help Babies

(As reported 2/25/99)
A study published Thursday warns mothers that getting pregnant too soon or waiting too long between having children can be bad for the new baby's health, reports CBS News Health Correspondent Dr. Emily Senay.

Mothers should wait at least 18 months after giving birth to get pregnant again, say researchers at the Centers for Disease Control and Prevention. Doctors conclude that spacing babies 2 and 1/2 years apart is best for their health.

"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 Thursday's New England Journal of Medicine.

The study was based on 173,205 births in Utah from 1989 to 1996. It found infants conceived 18 to 23 months after their mother's last birth were at lower risk of premature arrival and low birth weight than infants conceived less than six months or more than five years after the birth of a sibling.

Compared with babies born after the ideal interval, those whose mothers 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.

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.

Doctors compare the process to a runner training for a marathon. Once you've trained and run a marathon, it's good to take a break before the next one, but you don't want to wait too long or you'll lose the benefit of past training.

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.

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.

However, 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 Institute of Health cautioned that the researchers ight 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.