To examine whether risk factors differed among subgroups of preterm (<
37 weeks of gestation) deliveries, we studied a cohort of 1,825 enlist
ed servicewomen who delivered from 1987 through 1990 at four U.S. Army
medical centers. Preterm deliveries were classified by length of gest
ation (<29 weeks, 29-32 weeks, 33-36 weeks) and clinical course [medic
al indication, idiopathic preterm labor, or preterm rupture of membran
es (PROM)]. We abstracted medical records for information on age, race
, army rank, marital status, gravidity, parity, the baby's sex, matern
al prepregnancy height and weight, gestation at entry to prenatal care
, alcohol drinking and smoking, time since and outcome of preceding pr
egnancy, surgery performed during pregnancy, anemia, and diagnoses of
uterine abnormalities, sexually transmitted diseases, and urinary trac
t infections. We used proportional hazards analysis to evaluate associ
ations for each subgroup of preterm delivery. The relative odds associ
ated with a history of preterm delivery in the preceding pregnancy ran
ged from 3.1 for deliveries due to preterm labor or FROM to 6.2 for de
liveries that occurred during 29-32 weeks; none of the other factors w
as consistently associated across the subgroups of preterm delivery. T
he paucity of associations is consistent with the conclusion of other
investigators that most of the causes of preterm delivery are unknown.