**3.4 Valsalva forces**

Expulsive forces, or pushing through cognizant contraction of the striated muscles in the chest and abdomen, have been demonstrated to be necessary for achieving vaginal delivery [24]. Directed pushing, where the patient actively participates and follows verbal instruction, has been associated with a shorter duration of the second stage [6]. It is the cumulative effects of uterine contractions and expulsive forces that are necessary for the fetus to descend through the pelvis, resulting in a vaginal delivery.

We have found that patients with a BMI ≥ 35 kg/m<sup>2</sup> generate lower pressure while pushing in the second stage of labor [25]. Lower intrauterine pressure suggests less expulsive force and hence increased risk for second-stage dystocia or arrest. This suggests that obese patients may need a longer second stage of labor to achieve a vaginal birth.

There are several potential explanations for these observations. It is possible that the abdominal musculature in patients with a BMI ≥ 35 kg/m<sup>2</sup> is stretched beyond the optimal point of sarcomere overlap and therefore unable to generate adequate expulsive force through muscle contraction. Or perhaps, an increased body weight is also associated with metabolic dysregulation by which hyperlipidemia interferes with calcium influx and leads to fatty infiltration of the muscles, thus impairing abdominal and myometrial contractility. However, further research is needed to explore the exact mechanism.

Since obese patients can not generate sufficient expulsive force per contraction, it may be better to measure the cumulative force of sequential contractions combined with pushing generated during the second stage of labor as a parameter for assessing progress and determining the likelihood of achieving a successful vaginal delivery.
