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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
Preterm birth is the leading cause of perinatal morbidity and mortality in the United States (1).
10–15% of pregnancies experienced at least one episode of preterm labor.
10% of all births in the United States are preterm, of which 2/3 are spontaneous, and 1/3 are medically indicated.
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., urinary tract infection [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)
Familial predisposition. Numerous gene candidates mediating various pathways (inflammation, apoptosis, coagulation, hypoperfusion, thrombosis, collagen remodeling) have been identified, but causality and gene-environment interactions are not well-defined.
- Demographic factors, including single parent, poverty, and black race
- Short interpregnancy interval (<18 months)
- 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
- Routine transvaginal ultrasound cervical length (CL) measurements in singleton pregnancies in the mid-2nd trimester to detect increased risk for preterm birth (<20 mm) is an acceptable strategy (2) and may be especially helpful if risk factors for preterm delivery are present.
- Primary prevention:
- Interval contraception to optimize pregnancy spacing
- Smoking cessation
- If previous preterm birth, evaluate if etiology is likely to recur and target intervention to specific condition.
- Weekly injections of 17α-hydroxyprogesterone (250 mg IM every week) from 16 to 36 weeks if previous spontaneous preterm birth
- Consider cerclage (2)[A] or pessary placement (3)[B] before 24 weeks’ gestation for those at high risk because of cervical insufficiency or significant or progressive cervical shortening.
- Secondary prevention: