Association Between Interpregnancy Interval and Risk of Preterm Birth and Its Modification by Folate Intake: The Japan Environment and Children's Study.

BACKGROUND: Both short and long interpregnancy intervals (IPIs) have been associated with risk of preterm birth, but the evidence is limited in Asians. It is also uncertain whether the association is modified by dietary folate intake or folic acid supplementation during pregnancy. Thus, we examined associations between IPI and risk of preterm birth and effect modification of those associations by dietary intake of folate and supplementation with folic acid on the basis of a nationwide birth cohort study. METHODS: Among 103,062 pregnancies registered in the Japan Environment and Children's Study, 55,203 singleton live-birth pregnancies were included in the analysis. We calculated IPI using birth date, gestational age at birth of offspring, and birth data of the latest offspring. Odds ratios (ORs) and 95% confidence intervals (CIs) of the risk of preterm birth were estimated according to IPI categories. RESULTS: Both <6-month and ≥120-month IPIs were associated with an increased risk of preterm birth, compared with an 18-23-month IPI. The multivariable ORs were 1.63 (95% CI, 1.30-2.04) for <6-month and 1.41 (95% CI, 1.11-1.79) for ≥120-month IPIs. These associations were confined to women with inadequate intake of dietary folate and folic acid supplementation during pregnancy. Multivariable ORs were 1.76 (95% CI, 1.35-2.29) for <6-month IPI and 1.65 (95% CI, 1.24-2.19) for ≥120-month IPI. CONCLUSION: Both <6-month and ≥120-month IPIs were associated with an increased risk of preterm birth. These higher risks were confined to women with inadequate intake of dietary folate and folic acid supplementation during pregnancy.