Problems with External Monitors
The biggest advantage of cardiotocography (CTG) is that it is an external device. The disadvantage is its inability to give an accurate reading on intrauterine pressure. Without intrauterine pressure capability the CTG is not an indicated device for augmentation of labor. The CTG’s readings can not only be influenced by intra-amniotic pressure but by local uterine muscle tension and abdominal wall flexing, breathing, vomiting, coughing and normal movement of the mother. It is hampered by the thickness of the abdominal wall, which is why there is a high rate of failure in high BMI Moms.
Overall, in high BMI moms the accuracy in CTG is poor. Furthermore, the best position for CTG is supine, which is not desirable for the patient and limits mobility. The technology has remained essentially unchanged since 1968.
The problem with the qualitative CTG is its inability to determine the strength of the uterine contraction. The TrueLabor™ Maternal Fetal Monitoring System using LaborTrack™ Technology is able to give a quantitative assessment of how hard the uterus is contracting. Take a look at the videos below, both of which show CTG contractions peaking at approximately 60mm/Hg, but only the contraction on the right, as evaluated by TrueLabor™, results in an actual contraction. The one on the left looks like a contraction by the CTG but is actually only a “labor pain”.
The tocodynamometer presents no clear difference. The EUM displays 1 contraction. Similar electrical myometrial activity exists for each of the 3 contractions on the graph. The specific EUM representation is driven from the contraction annotated with a dot .
The TrueLabor™ Maternal Fetal Monitor Offers Accuracy in High BMI Moms
Does obesity affect the intensity of the electromyography signal, thus altering the predictive value for identification of the patient in preterm labor? We found no difference in signal:noise intensity regardless of maternal size. 
Published data supports that electrohysterographic contraction detection correlates with IUPC better than tocometry in patients with high body mass index. 
“. . . does obesity affect the intensity of the electromyography signal, thus altering the predictive value for identification of the patient in preterm labor? Euliano et al. Reported that electrohysterography had a stronger correlation to IUPC than tocodynamometry in obese patients. In contrast, we found no difference in the noise to signal intensity, regardless of maternal size. This disparity can be explained because of use of 9 electrodes and a sensor that enables the mapping of each electrode’s location within 1mm. This represents an accurate localization of the myometrial activity of the uterus in 3 dimensions. We observed that false contractions are characterized by sporadic activity, whereas active labor presents a much more synchronized behavior.” 
TOCO Fails Frequently in High BMI Moms
In a comparison of Toco monitoring in obese (BMI >35) and non-obese (BMI 20-25) women, Ray et al.  describe 30% rate of “difficult monitoring” in obese group (0% non-obese), requiring 26% rate of internal monitoring (0% non-obese).
Vanner et al.  similarly report poor quality Toco during more than one quarter of the monitoring time in 36% of obese parturients (16% non-obese).