research on the efficacy of antibiotic therapy for the prevention of preterm delivery from these infections continues.

Demographic risks associated with preterm delivery include African American race, single marital status, low socioeconomic status, maternal age, and others (Table B-6). Although demographic factors cannot cause the premature expulsion of a fetus, these factors may antagonize some other risk factor(s).

Social, behavioral, stress, and maternal psychological factors have frequently been linked to pregnancy outcomes; and chronic stress has been related to low socioeconomic status (57–66). There have been difficulties in measuring the amount of stress caused by life events, but consistent associations between perceived stress and preterm birth have been reported.