| Summary: | This case-control study aimed to test interaction between the most common sexually transmitted infections (STIs) (i.e., chlamydia, gonorrhea, and syphilis) and prenatal care (PNC) utilization status on preterm birth (PTB) (<37 weeks gestation) and low birthweight (LBW) (<2500 g). We used data of participants with singleton live births (N = 3,418,028) from the 2019 United States National Vital Statistics System. There were 280,206 participants in the PTB group and 3,137,822 in the control group, and 221,260 participants in the LBW group and 3,196,768 in the control group. Nearly 1.9% of the participants had chlamydia, 0.3% had gonorrhea, and 0.2% had syphilis. Interaction effects of STIs with PNC utilization status on the risk of PTB and LBW were tested on the multiplicative and additive scales. Using measures of the relative excess risk of interaction (RERI), the attributable proportion of interaction (AP), and the synergy index (S), we observed the highest significant synergistic interaction between syphilis and inadequate PNC utilization increasing the risk of PTB (RERI 2.12, AP 38%, and SI 1.88), and between gonorrhea and inadequate PNC utilization increasing the risk of LBW (RERI 1.03, AP 28%, and SI 1.64). Findings from this study help improve our understanding of disease etiology and inform prevention planning.
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