The interaction between no folic acid supplementation during early pregnancy and preeclampsia increased the risk of preterm birth.

Objectives: To explore if there is a positive additive interaction between no folic acid (FA) supplementation in early period of pregnancy and preeclampsia which increases the risk of preterm birth (PTB).

Methods: We matched 1471 women who had live-birth singleton preterm infants with 1471 women who had live-birth singleton term infants at 15 Chinese hospitals in 2018. We excluded women who took folic acid less than 0.4 mg/d or less than 12 weeks in early stage, women with gestational hypertension, chronic hypertension, or preeclampsia during previous pregnancy. We calculate odds ratios for PTB by performing conditional logistic regression comparing preterm group with term group.We quantified the interaction between 2 exposures by synergy (S) and relative excess risk due to interaction (RERI).

Results: Approximately 40% of preterm cases did not take FA in early pregnancy. After adjusting confounding factors by logistic regression, when the 2 exposures (no early FA supplementation and preeclampsia) co-existed, the risk of all PTB increased significantly (aOR11=12.138; 95% CI 5.726-25.73), the interaction between 2 exposures was positive (S=1.27) and increased 2.385-fold risk of all PTB (RERI=2.385); and there were similar results on iatrogenic PTB (aOR11=23.412; 95% CI 8.882-60.71, S=1.18, RERI=3.347).

Conclusion: Our multicenter study showed, for the first time, that there was a positive additive interaction between no FA supplementation in early pregnancy and preeclampsia which increased the risk of all PTB, especially iatrogenic PTB.