Medically induced preterm birth and the associations between prenatal care and infant mortality

During the last 30 years, the use of prenatal care, both the proportion of women receiving the recommended number of visits and the average number of visits, has increased substantially. Although infant mortality has decreased, the incidence of preterm birth has increased. We hypothesized that prenatal care may lead to lower infant mortality in part by increasing the detection of obstetrical problems for which the clinical response may be to medically induce preterm birth.

We examined whether medically induced preterm birth mediates the association between prenatal care and infant mortality by using newly developed methods for mediation analysis. Data are the cohort version of the national linked birth certificate and infant death data for 2003 births. Analyses were adjusted for maternal sociodemographic, geographic, and health characteristics.

Receiving more prenatal care visits than recommended was associated with medically induced preterm birth (odds ratio [OR], 2.44; 95% confidence interval [95% CI], 2.40-2.49) compared with fewer visits than recommended). Medically induced preterm birth was itself associated with greater infant mortality (OR, 5.08; 95% CI, 4.61-5.60) but that association was weaker among women receiving extra prenatal care visits (OR 3.08; 95% CI, 2.88-3.30) compared with women receiving the recommended number of visits or fewer.

These analyses suggest that some of the benefit of prenatal care in terms of infant mortality may be in part due to medically induced preterm birth. If so, the use of preterm birth rates as a metric for tracking birth policy and outcomes could be misleading.