Preterm Labor


Preterm birth is defined as birth between 20 0/7 and 36 6/7 weeks’ gestation.


Regular contractions occurring between 20 0/7 and 36 6/7 weeks’ gestation with either a change in effacement and cervical dilation or cervical dilation of ≥2 cm on presentation


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.
  • Causes of preterm births in the United States: spontaneous preterm labor makes up 40–45% of preterm births, preterm premature rupture of membranes makes up 30–35% of preterm births, medically indicated delivery due to maternal or neonatal etiology associated with 30–35% of preterm births, and multiple gestation


  • Non-Hispanic blacks are 50% more likely to have a preterm birth than Hispanic or white patients.
  • 10% of all births in the United States in 2019 were preterm: 50% of preterm deliveries are due to spontaneous preterm labor, 25% following preterm prelabor rupture of membranes, and 25% due to maternal or fetal medical indications due to complications (1).

Etiology and Pathophysiology

  • Premature formation and activation of myometrial gap junctions
  • Abnormal placental implantation/placental abruption/placental ischemic disease (preeclampsia and fetal growth restriction)
  • Systemic inflammation/infections (e.g., UTI, autoimmune conditions)/immunopathology (e.g., antiphospholipid antibodies)
  • Local inflammation/infections (e.g., intraamniotic infections, group B Streptococcus [GBS])—colonization with infectious organism noted in 25–40% of all preterm births
  • Uterine abnormalities/overdistension (multiple gestation, polyhydramnios)/cervical insufficiency/premature dilation
  • Preterm premature rupture of membranes
  • Trauma
  • Fetal abnormalities
  • Hypertensive disorders of pregnancy

Familial predisposition; numerous gene candidates mediating various pathways have been identified, but causality and gene–environment interactions are not well-defined.

Risk Factors

  • Prior preterm delivery (most significant risk factor): >3-fold if previous preterm birth
  • Demographic factors, including social and economic disadvantages and black race with concern for chronic stress from structural racism
  • Short interpregnancy interval (<18 months)
  • Prepregnancy weight <45 kg (<100 lb), body mass index <18.5
  • Substance abuse (e.g., cocaine, tobacco)
  • Unintended pregnancy
  • Cervical insufficiency/short cervical length of <25 mm
  • Abdominal surgery/trauma during pregnancy; history of dilation and curettage
  • Uterine structural abnormalities, such as large fibroids or müllerian abnormalities
  • Serious maternal infections/diseases; bacterial vaginosis and genital tract infections; bacteriuria
  • Multiple gestation; intrauterine growth restriction; polyhydramnios
  • Placenta previa; vaginal bleeding; premature placental separation (abruption)

General Prevention

  • 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
  • For women with a singleton pregnancy and no history of spontaneous preterm births, visualization of the cervix on routine fetal anatomical scan between 18 0/7 and 22 6/7 weeks’ gestation should be performed. If found to be shortened, a transvaginal ultrasound should be done for further evaluation (1)[C]. This recommendation is the same for multiple gestation and women with a history of medically induced preterm birth. Twin pregnancies are more likely to have a shortened cervix during the 2nd trimester, and women with a history of medically induced preterm birth are more likely to have a subsequent preterm birth (1).
  • For women with history of spontaneous preterm birth and singleton pregnancy, a screening transvaginal ultrasound to evaluate cervix length should be done at 16 0/7 weeks’ gestation and repeated every 1 to 4 weeks (protocols vary) until 24 0/7 weeks (1).
  • Primary prevention involves counseling on: interval contraception to optimize pregnancy spacing and smoking cessation; if previous preterm birth, evaluate if etiology is likely to recur and target intervention to specific condition. Women with history of spontaneous preterm birth and singleton pregnancy should be offered vaginal or intramuscular progesterone to prevent preterm birth with a shared decision model (1).
  • Secondary prevention:
    • For patients with a cervix of <25 mm in length with or without a history of previous spontaneous preterm delivery and singleton pregnancy, it has been recommended to give vaginal progesterone if not already on progesterone. Most protocols give 200 mg/day at diagnosis at 18 0/7 to 25 6/7 weeks’ gestation to continue until 36 to 37 weeks (1). An FDA advisory committee voted in late 2022 to withdraw approval of 17-hydroxy progesterone and its generic equivalents based on results from postmarket confirmatory trial data in the PROLONG study, released in October 2019, showing lack of efficacy. Data is insufficient to recommend vaginal progesterone for multiple gestation (1).
    • Women with a singleton pregnancy and history of spontaneous preterm birth with shortened cervix should be considered for a cerclage (1). Cerclage can be considered for any patient who is noted to have cervical insufficiency on physical exam (1).
  • Tocolysis

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