Risk of spontaneous preterm birth in relation to maternal characteristics (n = 34,480).

HRs adjusted for maternal age, body mass index, race and ethnicity, educational level, marital status, smoking, parity, and history of prior preterm birth and preconceptional folate supplementation. Categorical variables: age, 30; married, not married (reference) versus married; education, ��12 (reference) versus >12 y; smoking, nonsmoking (reference) versus smoking; nulliparous, nulliparous versus parous (reference); prior preterm birth, no prior preterm birth (reference) versus prior preterm birth. Proportional hazard assumption was met for each characteristic (proportional hazard test, p>0.05 for all).an = Number of spontaneous preterm births in each category in order as above or for dichotomous categories in order yes no.