Description

Contractions occurring between 20 and 36 weeks’ gestation at a rate of 4 in 20 minutes or 8 in 1 hour with at least 1 of the following: Cervical change over time or dilation ≥2 cm

Epidemiology

Preterm birth is the leading cause of perinatal morbidity and mortality in the US.

Incidence
10–15% of pregnancies experience at least 1 episode of preterm labor.

Prevalence
~12% of all births in the US are preterm (9% spontaneous preterm births and 3% indicated preterm births).

Risk Factors

• 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 LEEP)
• Abdominal surgery/trauma during pregnancy
• Uterine anomalies, such as large fibroids or müllerian abnormalities
• Serious maternal infections/diseases
• Bacterial vaginosis
• Bacteriuria
• Vaginal bleeding during pregnancy
• Multiple gestation
• Select fetal abnormalities
• Intrauterine growth restriction
• Placenta previa
• Premature placental separation (abruption)
• Polyhydramnios
• Ehlers-Danlos syndrome

Genetics
Familial predisposition

General Prevention

• Patient education at each visit in 2nd and 3rd trimesters for those at risk and periodically in the last 2 trimesters for the general population
• 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 16–36 weeks if previous spontaneous preterm birth (1,2)[A]
  • Consider cerclage placement before 24 weeks' gestation for those at high risk because of cervical insufficiency or significant or progressive cervical shortening.
• For women with a short cervix in the 2nd trimester (<20 mm on transvaginal US), progesterone 200 mg/d per vagina × 24–34 weeks may decrease the risk of preterm delivery (3,4)[A].

Pathophysiology

• Premature formation and activation of myometrial gap junctions
• Inflammatory mediator–stimulated contractions
• Weakened cervix (structural defect or extracellular matrix defect)
• Abnormal placental implantation

Etiology

• 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, septa, diethylstilbestrol exposure)
• Overdistension (by multiple gestation or polyhydramnios)
• Preterm premature rupture of membranes
• Trauma
• Placental abruption
• Immunopathology (e.g., antiphospholipid antibodies)
• Placental ischemic disease (preeclampsia and fetal growth restriction)