Uterine activity thresholds for Montevideo units, contraction frequency, and baseline tone predict neonatal acidemia

Monitoring uterine activity (UA) during labor is critical to ensure adequate contractions for progression while preventing fetal distress. Excessive or poorly managed UA can lead to neonatal acidemia, a condition characterized by low umbilical artery pH, which is associated with adverse outcomes like cerebral palsy and even death. Current guidelines for defining normal versus excessive UA often lack precise, evidence-based thresholds, making clinical decision-making challenging. This study aims to identify specific, clinically relevant UA thresholds that correlate with the risk of neonatal acidemia, particularly in the context of oxytocin-stimulated labors.

This blinded secondary analysis included 625 term singleton pregnancies from a completed randomized controlled trial (NCT02941393). Participants had intrauterine pressure (IUP) monitoring, cephalic presentation, and umbilical cord blood sampling at birth. Measures included IUP expressed as Montevideo units (MVUs), contraction frequency per 10 min, and uterine baseline tone. Associations between UA parameters and umbilical artery (UmA) pH (≤7.15) or base excess were assessed using linear and logistic regression and Student's t-test.

Elevated uterine activity parameters were significantly associated with an increased risk of neonatal acidemia. With an IUP cutoff >250 MVUs, the proportion of neonates with pH ≤7.15 increased from 16.8% to 25.6% (OR 1.68, 95% CI 1.10-2.59; P = 0.017). Among oxytocin-stimulated labors (n = 579), a lower IUP cutoff >200 MVUs was associated with an increase in pH ≤7.15 from 14.7% to 21.4% (OR 1.58, 95% CI 1.02-2.44; P = 0.038). Elevated baseline tone also showed a similar association, with the incidence of pH ≤7.15 increasing from 16.6% to 23.6% at a threshold >20 mmHg (OR 1.59, 95% CI 1.06-2.39; P = 0.025). A significant downward shift in UmA pH distribution was observed at a contraction frequency >5/10 min (7.23 [0.09] vs. 7.22 [0.09]; P = 0.042). For oxytocin-stimulated labors, this shift occurred at a lower contraction frequency >4/10 min (7.24 [0.08] vs. 7.23 [0.09]; P = 0.016).