Interpregnancy Interval After Termination of Pregnancy and the Risks of Adverse Outcomes in Subsequent Birth.

OBJECTIVE: To assess whether the length of the interpregnancy interval after termination of pregnancy influences the risk of preterm birth, low birth weight, and small-for-gestational-age neonates in a subsequent pregnancy. METHODS: In this register-based study, we included all women (N=19,894) who underwent termination of pregnancy between 2000 and 2009 and whose subsequent pregnancy ended in live singleton delivery. The women were divided into five groups depending on the interpregnancy interval between termination of pregnancy and subsequent conception: interpregnancy interval less than 6 months (n=2,956), 6 to less than 12 months (n=3,203), 12 to less than 18 months (n=2,623), 18 to less than 24 months (n=2,076), and 24 months or greater (n=9,036). The incidences and unadjusted and adjusted risks of preterm birth, low birth weight, and small-for-gestational-age neonates were calculated in relation to the different interpregnancy interval lengths, the reference group being that with an interpregnancy interval of 18 to less than 24 months. RESULTS: There was a significant difference in the rate of preterm birth between the group with the interpregnancy interval less than 6 months and the reference group (5.6% compared with 4.0%, respectively, P=.008). After adjusting for nine background factors, an interpregnancy interval of less than 6 months was associated with an increased risk of preterm birth (adjusted odds ratio 1.35, 95% confidence interval 1.02-1.77). No such association emerged in longer interpregnancy interval groups or regarding other adverse events. The possibility of unmeasured confounding cannot be ruled out. CONCLUSION: Slightly but significantly increased estimated risk of preterm delivery in subsequent pregnancy was seen when the interpregnancy interval after termination of pregnancy was less than 6 months. These data emphasize the need for prompt initiation of effective contraception after termination and enable counseling the patient for optimal conception interval.