Preterm Labor

Risk factors

• Prior spontaneous preterm birth (strongest predictor)
• Short cervix on transvaginal ultrasound (<25 mm before 24 wk)
• Multiple gestation, uterine anomaly (septate, bicornuate uterus)
• Infection (UTI, BV, periodontal disease, intra-amniotic infection)
• Smoking, substance use, short interpregnancy interval, low BMI, Black race, low socioeconomic status

Pathophysiology

Final common pathway involves activation of decidua, myometrium, and cervix. Triggers include inflammation/infection, uteroplacental ischemia, uterine overdistension, decidual hemorrhage, and stress. Progesterone withdrawal at the local level, prostaglandin release, and oxytocin receptor upregulation drive coordinated contractions and cervical ripening.

Clinical presentation

Differential diagnosis

• Braxton-Hicks contractions — Irregular, painless, not associated with cervical change; resolve with hydration and rest
• Round ligament pain — Sharp groin pain with movement; not rhythmic; no cervical change
• UTI / pyelonephritis — Dysuria, frequency, CVA tenderness; UA positive; can precipitate preterm contractions
• Placental abruption — Painful bleeding with rigid 'board-like' uterus; FHR abnormalities
• Constipation / GI distress — Abdominal cramping not localized to uterus; no cervical change

Diagnostic workup

Diagnostic algorithm

flowchart TD
  A[Contractions <37 wk] --> B[Speculum + TVUS cervical length]
  B --> C{Cervical change<br/>or CL <20 mm?}
  C -->|No, CL >=30 mm| D[Likely false labor<br/>discharge with precautions]
  C -->|Equivocal 20-29 mm| E[Fetal fibronectin]
  E -->|Negative| D
  E -->|Positive| F[Admit, treat as PTL]
  C -->|Yes| F
  F --> G[Antenatal steroids<br/>24-34 wk]
  F --> H[Tocolysis x 48 h<br/>nifedipine or indomethacin]
  F --> I[MgSO4 neuroprotection<br/>if <32 wk]
  F --> J[GBS prophylaxis<br/>if delivery imminent]

Treatment

Complications

• Preterm birth with prematurity-related morbidity: RDS, BPD, IVH, NEC, retinopathy of prematurity, neurodevelopmental impairment
• Maternal: tocolytic side effects (pulmonary edema with beta-agonists, hypotension with nifedipine, ductal closure with indomethacin)
• Magnesium toxicity (loss of DTRs, respiratory depression) — treat with IV calcium gluconate

PANCE pearls

• Indomethacin tocolysis is contraindicated after 32 wk because it can cause premature ductal closure in utero and oligohydramnios.
• Avoid combining magnesium sulfate with nifedipine due to additive hypotension and neuromuscular weakness.
• Fetal fibronectin is most useful for its strong negative predictive value — a negative fFN supports outpatient management.
• Magnesium for neuroprotection (<32 wk) is given regardless of whether the patient is also receiving it as a tocolytic.
• Cervical length <25 mm + prior spontaneous PTB is the cerclage indication; isolated short cervix without prior PTB is treated with vaginal progesterone, not cerclage.

References

• ACOG PB 234 — ACOG Practice Bulletin 234: Prediction and Prevention of Spontaneous Preterm Birth (Obstet Gynecol 2021)
• ACOG PB 171 — ACOG Practice Bulletin 171: Management of Preterm Labor (Obstet Gynecol 2016)
• PROLONG — Blackwell et al., Am J Perinatol 2020 — 17-OHPC efficacy
• BEAM Trial — Rouse et al., NEJM 2008 — MgSO4 for fetal neuroprotection