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Contractions occurring between 20 and 36 weeks’ gestation at a rate of 4 in 20 minutes or 8 in 1 hour with at least one of the following: cervical change over time or dilation ≥2 cm (1)
Preterm birth is the leading cause of perinatal morbidity and mortality in the United States.
10–15% of pregnancies experienced at least one episode of preterm labor.
~12% of all births in the United States are preterm (9% spontaneous preterm births and 3% indicated preterm births).
Etiology and Pathophysiology
- Premature formation and activation of myometrial gap junctions
- Inflammatory mediator–stimulated contractions
- Weakened cervix (structural defect or extracellular matrix defect)
- Abnormal placental implantation
- Systemic inflammation/infections (e.g., UTI, pyelonephritis, pneumonia, sepsis)
- Local inflammation/infections (intra-amniotic infections from aerobes, anaerobes, Mycoplasma, Ureaplasma)
- Uterine abnormalities (e.g., cervical insufficiency, leiomyomata, müllerian anomalies, diethylstilbestrol exposure)
- Overdistension (by multiple gestation or polyhydramnios)
- Preterm premature rupture of membranes
- Placental abruption
- Immunopathology (e.g., antiphospholipid antibodies)
- Placental ischemic disease (preeclampsia and fetal growth restriction)
- Demographic factors, including single parent, poverty, and black race
- Short interpregnancy interval
- No prenatal care
- Prepregnancy weight <45 kg (100 lb), body mass index <20
- Substance abuse (e.g., cocaine, tobacco)
- Prior preterm delivery (common)
- Previous 2nd-trimester dilation and evacuation (D&E)
- Cervical insufficiency or prior cervical surgery (cone biopsy or loop electrosurgical excision procedure [LEEP])
- Abdominal surgery/trauma during pregnancy
- Uterine structural abnormalities such as large fibroids or müllerian abnormalities
- Serious maternal infections/diseases
- Bacterial vaginosis
- Vaginal bleeding during pregnancy
- Multiple gestation
- Select fetal abnormalities
- Intrauterine growth restriction
- Placenta previa
- Premature placental separation (abruption)
- Ehlers-Danlos syndrome
- Patient education at each visit in 2nd and 3rd trimesters for those at risk and periodically in the last two trimesters for the general population
- If previous preterm birth, evaluate if etiology is likely to recur and target intervention to specific condition.
- For women with a short cervix in the 2nd trimester (<20 mm on transvaginal US), progesterone 200 mg/day per vagina for 24 to 34 weeks may decrease the risk of preterm delivery (3)[A].