**3.2 Prolonged second stage of labor**

A prolonged second stage of labor refers to a delay in the progression of labor following complete cervical dilation, which has been redefined by ACOG in recent years to prevent primary cesarean sections. Unfortunately, obesity is recognized as one of the leading causes of a protracted labor course, as studies indicate that obese patients take approximately 1.3 hours longer to progress from 4 to 10 cm [1, 11–13]. While it is well-established that the first stage of labor is prolonged in obese patients, the

duration of the second stage remains a subject of controversy [13]. Some studies suggest a longer duration [14, 15], others indicate equivalence [16, 17], and others report a shorter duration of the second stage [18]. This variability could be attributed to factors such as the increased likelihood of cesarean delivery interrupting the natural course of labor or the wide range of BMI values encompassed within these studies. Here we approach this issue from a different perspective.

Traditionally, the duration of the second stage of labor is measured in length of time. Defining the start of the second stage based on the cervix reaching 10 cm is prone to inaccuracies due to the variability in frequency of pelvic examination during labor. It is entirely possible that many patients are completely dilated well before the examiner detects it. The length of the second stage also depends on the frequency of uterine contractions, the time a patient spent "laboring down", and any breaks they may take in between or during contractions. Thus, defining a prolonged second stage by the total length of time is confounded by the heterogeneity of the clinical picture. Medical decision-making based on absolute time definitions may even result in patients undergoing cesarean delivery before having an adequate chance or length of time to push. Therefore, we propose using the cumulative length of time a patient is pushing or the number of pushes as a clinical measurement to define the optimal length of the second stage.

Concerns have been raised about the impact of a prolonged second stage of labor on perinatal outcomes. A randomized control trial conducted by Gimovsky et al. demonstrated that an extra hour of pushing reduced the incidence of cesarean delivery from 43.2 to 19.5% without increasing maternal and neonatal morbidity [19]. There have been several studies including a large multicenter randomized control trial, cohort studies, and systematic reviews that have also shown no increase in perinatal outcomes with a prolonged second stage of labor [20–22]. Therefore, it may be reasonable to extend the second stage pushing time for patients without fetal heart rate abnormalities, especially for obese patients [4].

### **3.3 Uterine contractility**

Uterine contractions are essential to descent and ultimately delivery of the fetus. Contractions exhibit three main properties: frequency, duration, and strength [23]. In the second stage of labor, it is observed that contractions last up to 1 minute and recur at intervals no longer than 90 seconds [10]. Obesity is linked to reduced uterine contractility and irregular contraction patterns [11, 14]. While the exact mechanism for dysfunctional contractions in obese patients is unknown, there are several potential causes. *In vitro* myometrial samples from obese patients demonstrate a decrease in intracellular calcium influx during a contraction [11]. Impaired myometrial contractility may also be due to hyperlipidemia in obese patients, as oxytocin receptors are located near cholesterol-rich regions of the cell membrane known as lipid rafts which may be influenced by local lipid accumulation [11].

Despite dysfunctional contractions, obese patients are still capable of generating adequate Montevideo units, but may require higher doses of oxytocin to produce adequate contractions [17]. Obese individuals undergoing labor induction require progressively higher cumulative doses of oxytocin as their BMI class increases [17]. In order to optimize the chances of a vaginal delivery in obese patients, it may be necessary to closely monitor and aggressively adjust oxytocin doses to ensure adequate contractions to facilitate progression of labor.
