Interpregnancy interval and the risk of premature infants.

OBJECTIVE: Interpregnancy intervals are associated with the risk of low birth weight (LBW) infants, but the association between interpregnancy interval and prematurity is unknown. Our objective was to determine whether interpregnancy intervals were associated with the risk of premature infants and to define the degree of risk according to interpregnancy interval.
METHODS: We analyzed 289,842 singleton infants born to parous Mexican-origin Hispanic and non-Hispanic white women in the United States who resided in the same county and delivered between January 1, 1991 and September 30, 1991. Interpregnancy interval was defined as the number of months between the previous live birth and conception of the index pregnancy. Multivariate logistic regression analysis was used to estimate odds ratios and 95% confidence intervals for the risk of interpregnancy interval on very premature (23-32 weeks), moderately premature (33-37 weeks), and term gestation (38-42 weeks).
RESULTS: Nearly 37% of women had interpregnancy intervals less than 18 months, 45.5% of women had intervals of 18-59 months, and 17.6% of women had intervals over 59 months. After adjusting for confounding variables, women with intervals less than 18 months were 14-47% more likely to have very premature and moderately premature infants than women with intervals of 18-59 months. Women with intervals over 59 months were 12-45% more likely to have very premature and moderately premature infants than women with intervals of 18-59 months.
CONCLUSION: Women with interpregnancy intervals from 18-59 months had the lowest risk of very premature and moderately premature infants. Further study is needed to define the mechanisms through which interpregnancy interval influences pregnancy outcome.