Conceiving a subsequent child 18 to 23 months after a live birth may provide the ideal conditions for having a healthy, full term baby, according to a study published in the New England Journal of Medicine (1999;340:589-94).
The study, conducted by the United States Centers for Disease Control and Prevention, found that although having babies too close together may be associated with an adverse effect on an infant's health, having them too far apart may be even worse.
Compared with babies conceived 18 to 23 months after a live birth, babies conceived within six months had a 40%greater chance of being premature or undersized at delivery. Women who waited 10 years before having another child were twice as likely to have a small baby and 50%more likely to deliver prematurely.
Dozens of studies have linked short interpregnancy intervals with a higher risk of small and premature infants but none has determined the optimal interval. In the latest study, researchers evaluated interpregnancy interval in relation to low birth weight, preterm birth, and small size for gestational age in 173205 singleton infants born alive to multiparous mothers in Utah from 1989 to 1996. They controlled for 16 factors that could affect outcomes including smoking, drinking, prenatal care, and mother's age.
The relation between short intervals and adverse outcomes has been attributed to maternal nutritional depletion, damage to the reproductive system, and postpartum stress. It is unknown, however, why a long interpregnancy interval is also associated with adverse perinatal outcomes.
The authors of the study suggested that the mother's body becomes primed for birth during the earlier pregnancy, with enlargement of the uterus and an increase in blood flow to the uterus but these benefits decline over time. Informing mothers about these findings could help reduce health complications in babies, they said.