Correlation of external and internal monitoring of uterine activity in a cohort of term patients

External tocodynamometry (ET) and intrauterine pressure monitoring (IUPC) are commonly used to assess uterine contractile activity. It is believed that ET does not accurately document contraction amplitude. However, no direct comparison of ET and IUPC has been published in term patients. Our objective was to determine the correlation between frequency, duration, and amplitude as measured by ET and IUPC. Twenty patients between 36 and 41 weeks' gestation were enrolled during active spontaneous labor. Uterine contractions were recorded simultaneously by ET and IUPC for 2 hours. ET and IUPC data were compared using the Spearman correlation coefficient and the Wilcoxon signed-rank test. The median body mass index (BMI) of the patient population was 31.8 kg/m(2) (range 22.2 to 42.3). There was good correlation of contraction frequency measured by ET and IUPC (r = 0.75, p = 0.001), but poor correlation of both contraction amplitude (r = 0.26) and duration (r = 0.27). ET and IUPC assessment of uterine contractile activity, specifically contraction amplitude and duration, do not appear to correlate in a population of moderately obese patients. This suggests that ET should not exclusively be relied upon in this population to assess uterine contractile activity during labor.