Contraction frequency and acidemia at birth: A case-control study

Consensus is lacking regarding the upper limit for a safe contraction frequency during labor. We aimed to assess the association between different contraction frequencies and acidemia at birth, and whether an association was affected by oxytocin augmentation, concomitant cardiotocography (CTG) classification, and stage of labor.

This is a case-control study based on CTG traces from births in southern Sweden during 2012-2017. Cases (N = 364) had umbilical artery pH <7.1 if born by first-stage cesarean section and pH <7.05 if born vaginally or by second-stage cesarean. Controls were included with a 1:2 ratio (N = 728) and had pH ≥7.15. CTG traces from the hour prior to birth were assessed blinded to the clinical outcome. Logistic regression was used to estimate odds ratios (OR) with 95% confidence intervals (CI).

After excluding insufficient topography traces, 328 cases and 677 controls remained for analysis. Contraction frequencies 5, 5-6, and ≥6 per 10 min occurred in 15.2%, 19.5%, and 29.6% of cases and in 15.1%, 16.5%, and 11.1% of controls, respectively. The OR (95% CI) for acidemia were 1.6 (1.1-2.4), 1.9 (1.3-2.7), and 4.3 (3.0-6.2), respectively, compared with a reference group of <5. When comparing two separate binary cut-offs of >5 and ≥5 contractions, similar strengths of association with acidemia were demonstrated with OR 2.7 (2.1-3.6) and 2.5 (1.9-3.3), respectively. The effect of contraction frequency >5 was similar in spontaneous (n = 586) and oxytocin-augmented (n = 419) labor (OR 2.5, 1.7-3.8 vs. OR 2.4, 1.6-3.6), and whether the CTG was normal, suspicious, or pathological according to the Swedish CTG template (OR 2.1, 1.3-3.3; OR 3.5, 1.6-7.6; and OR 2.6, 1.4-4.7, respectively). The study lacked power to study any association during the first stage of labor.

Increasing contraction frequency is incrementally associated with fetal acidemia. Although an increased risk of acidosis is evident at contraction frequencies as low as five, such frequencies are common while acidemia is uncommon, implying a poor positive predictive value. A contraction frequency of more than five is associated with acidemia in both spontaneous and oxytocin-augmented labor, and even when the fetal heart trace is normal.