Description

Regular contractions occurring between 20 and 36 weeks and 6 days with either a change in effacement and cervical dilation or cervical dilation of ≥2cm on presentation.

Epidemiology

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

Incidence
10–15% of pregnancies experienced at least one episode of preterm labor.

Prevalence
10.23% of all births in the United States in 2019 were preterm (1). Non-Hispanic blacks are 50% more likely to have a preterm birth than Hispanic or white patients (1).

Etiology and Pathophysiology

• Premature formation and activation of myometrial gap junctions
• Abnormal placental implantation
• Systemic inflammation/infections (e.g., UTI)
• Local inflammation/infections (e.g., intra-amniotic infections)
• Uterine abnormalities
• 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)

Genetics
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)
• 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
• History of dilation and curettage
• Cervical insufficiency
• Short cervical length of <25 mm
• Abdominal surgery/trauma during pregnancy
• Uterine structural abnormalities, such as large fibroids or müllerian abnormalities
• Serious maternal infections/diseases
• Bacterial vaginosis
• Bacteriuria
• Multiple gestation
• Intrauterine growth restriction
• Placenta previa
• Premature placental separation (abruption)
• Polyhydramnios

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
• For women with a singleton pregnancy and no history of spontaneous preterm births, visualization of the cervix on routine fetal anatomical scan at 18 0/7 and 22 6/7 should be performed. If found to be shortened, a transvaginal ultrasound should be done for further evaluation (2)[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 (2).
• 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 and repeated until 24 0/7 weeks. Many protocols will repeat every 1 to 4 weeks during this time (2)^.
• 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.
  • 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 (2).
• Secondary prevention:
  • For patients with a cervix of <25 mm with or without a history of previous spontaneous preterm delivery and singleton pregnancy, it is recommended to give vaginal progesterone if not already on progesterone. Most protocols give 200 mg daily at diagnosis at 18 0/7 to 25 6/7 to continue until 36 to 37 weeks (2).
  • Women with a singleton pregnancy and history of spontaneous preterm birth with shortened cervix should be considered for a cerclage (2).
  • Data is insufficient to recommend vaginal progesterone for multiple gestation (2).
  • Cerclage can be considered for any patient who is noted to have cervical insufficiency on physical exam (2).
  • It is not recommended to do a cervical pessary in any patient with shortened cervix (2).
• Tocolysis