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The number of premature infants born in the United States in the past decade increased by 8 percent. These infants suffer from more health problems and higher mortality rates than infants born at term. Previous studies have shown that short intervals between pregnancies are associated with a higher risk of low birthweight, but the effects on prematurity are unknown.
A recent study by researchers at the Medical Effectiveness Research Center for Diverse Populations at the University of California, San Francisco, shows that women with interpregnancy intervals from 18 to 59 months have the lowest risk of delivering premature infants. The study was supported in part by the Agency for Healthcare Research and Quality (HS07373).
Women whose interpregnancy interval (interval between the end of one pregnancy and conception of the next one) is between 18 and 59 months have the lowest risk of giving birth to very premature (23-32 weeks' gestation) and moderately premature (33-37 weeks' gestation) infants, concludes this study. The researchers analyzed 289,842 single infants born in the first 9 months of 1991 to Mexican-American and non-Hispanic white women who lived in the same U.S. county. Nearly 37 percent of the women had interpregnancy intervals less than 18 months, 45.5 percent had intervals of 18 to 59 months, and 17.6 percent had intervals over 59 months.
After adjusting for demographic, obstetric, and health-service factors affecting prematurity, the researchers found that women with less than 18 months between pregnancies were 14 to 47 percent more likely to have very premature and moderately premature infants than women with 18 to 59 months between pregnancies. Women with intervals over 59 months were 12 to 45 percent more apt to have very premature and moderately premature infants than women with intervals of 18 to 59 months. Interpregnancy intervals less than 18 months were more strongly associated with very premature infants than with moderately premature infants. Women who had less than 6 months between pregnancies had the highest risk of delivering premature infants.
Women with short intervals between pregnancies may have more nutritional deficiencies than women with longer intervals because of the shorter time to replenish their bodies between pregnancies. This group of women also may have more behavioral risk factors, such as tobacco or alcohol use, than women with longer intervals. Finally, women with short intervals are thought to experience more psychological or emotional stress than women with longer intervals. The authors call for studies to analyze the behavioral, nutritional, and psychological factors associated with interpregnancy intervals and prematurity.
Details are in "Interpregnancy interval and the risk of premature infants," by Elena Fuentes-Afflick, M.D., M.P.H., and Nancy A. Hessol, M.S.P.H., in the March 2000 Obstetrics & Gynecology 95(3), pp. 383-390.
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