Maternal characteristics

Black race

Interpregnancy interval of less than six months

Physically strenuous or stressful work

Prepregnancy body mass index = 19 kg per m2

Pregnancy history

Previous preterm delivery

Pregnancy characteristics

Bacterial vaginosis, Chlamydia infection

Cocaine or heroin use

History of cervical cone biopsy or loop electrosurgical excision procedure

Intrauterine infection

Maternal abdominal surgery

Maternal medical disorders such as thyroid disease, diabetes mellitus, or hypertension

Multiple gestation

Nongenital tract infection (asymptomatic bacteriuria, pneumonia, appendicitis)

Periodontal disease

Polyhydramnios or oligohydramnios

Shortened cervix (< 3.0 cm)

Tobacco use

Uterine anomalies

Vaginal bleeding caused by placental abruption or placenta previa