Differences between external and internal fetal heart rate monitoring during the second stage of labor: a prospective observational study

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To compare fetal heart rate (FHR) signals acquired simultaneously by an external ultrasound probe and a scalp electrode during the second stage of labor.


This was a prospective observational study in a labor ward of a tertiary care university hospital. The population was women in labor with uneventful singleton pregnancies at term. Simultaneous external and internal FHR monitoring was performed in 67 consecutively recruited women during the second stage of labor. Cases were subsequently excluded if the trace length was under 40 min, cesarean birth occurred, or the interval between trace-end and birth exceeded 5 min, leaving a total of 33 traces for analysis. The last 40–60 min of these traces were analyzed by a computer system (Omniview-SisPorto® 3.5; Speculum®, Lisbon, Portugal) to quantify cardiotocographic parameters. Paired sample t-test and Bland-Altman limits of agreement (LoA) were used for statistical analysis, setting significance at 0.05. The main outcome measures were signal loss, FHR baseline, periodic events, and percentage of periodic events coinciding with contractions.


A higher signal loss was observed with external monitoring [10% vs. 4%; P<0.001, LoA=(-6, 18)]. No differences were found in mean FHR baseline [129 bpm vs. 130 bpm, P=0.245, LoA=(-15, 12)], but more accelerations [12 vs. 8, P<0.001, LoA=(-5, 13)] and less decelerations [8 vs. 10, P<0.001, LoA=(-8, 4)] were detected with external monitoring. With this method there were also more accelerations (66% vs. 55%, P=0.036) and less decelerations (68% vs. 81%, P=0.017) coinciding with contractions.


External FHR monitoring during the second stage of labor results in higher signal loss, increased number of accelerations, and decreased number of decelerations when compared with internal monitoring.

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