**2. Pelvic anatomy and physiology of labor**

#### **2.1 Anatomy**

The maternal pelvis consists of the bony pelvis and surrounding soft tissue structures. The size and shape of the pelvis play an important role in the second stage of labor. Four different pelvic types have been described (**Figure 1**): gynecoid (41.4%), android (32.5%), anthropoid (23.5%), and platypelloid (2.6%) [5]. The most common pelvic type is gynecoid, which is considered the most favorable shape for fetal descent during the different cardinal movements of labor (engagement, descent, flexion, internal rotation, extension, external rotation). During the second stage of labor, the fetal head is mostly found in an oblique occiput anterior (OA) position [6]. However, certain pelvis types like anthropoid and platypelloid

#### **Figure 1.**

*Illustrating the four different pelvic types, the gynecoid pelvis is considered the most ideal type for childbirth due to its wide pelvic inlet. The android pelvis, more commonly found in males but occasionally in females, has a heartshaped pelvic inlet with a narrow opening and a pronounced sacrum. The anthropoid pelvis is characterized by an oval-shaped inlet and is often associated with the OP position of the baby's head during labor. Lastly, the platypelloid pelvis is wide and flat in shape.*

*Second Stage of Labor in Obese Patients: Calling for a New Definition DOI: http://dx.doi.org/10.5772/intechopen.112785*

are at a higher risk of fetal malposition, such as occiput posterior (OP) and occiput transverse (OT) position, respectively. Patients with a fetus in the OP position have been found to have a longer second stage of labor, up to 60 minutes longer compared to a fetus in the OA position [7]. A systematic review of randomized controlled trials demonstrated that manual rotation of the occiput was associated with a higher rate of vaginal delivery: 64.9% in the manual rotation group vs. 59.5% in the control group [8]. Manual rotation also may shorten the second stage of labor by 15 to 30 minutes, depending on parity [8]. Manual rotation causes no difference in neonatal outcomes and thus should be attempted before proceeding to operative delivery or cesarean delivery [4].
