Over 95 percent of fetuses are in cephalic presentation at term. The position of the fetal occiput can be anterior, transverse or posterior (figure 1A-C). Fifteen to 20 percent of term fetuses are in occiput posterior (OP) position before labor [1,2]. Most of these fetuses rotate intrapartum: the incidence at vaginal birth is approximately 5 percent. Persistence of the OP position is important because it can be associated with labor abnormalities and maternal and neonatal complications (eg, birth trauma, neonatal acidosis) .
It had been assumed that the OP position at birth resulted from failure of an OP fetus to spontaneously rotate to occiput anterior (OA) position. Sonographic studies of fetal position have challenged this assumption and suggested that the OP position is actually often the result of malrotation from an OA position. As an example, a study that performed ultrasounds on 270 cephalic fetuses ≥36 weeks of gestation prior to induction found that 17 of 25 fetuses delivered from the OP position had been in a non-OP position prior to labor .
RISK FACTORS AND CONSEQUENCES
Risk factors for OP position at delivery include [4-9]:
●Maternal age greater than 35 years