Face and Brow Presentation Deformation

GENESIS

In face and brow presentations, the face is the compressed presenting part, usually with extension of the head ( Fig. 44.1 ). Face presentation is an abnormal cephalic fetal presentation where the presenting part is the mentum, occurring in 1 to 2 per 1250 deliveries. In persistent brow presentation , the neck is not extended as much as in face presentation, with the leading part of the face being the area between the anterior fontanelle and the orbital ridges. A brow presentation is less common and occurs in 1 in 500 to 1 in 4000 deliveries. Anything that delays or prevents flexion such as fetal anomalies, contracted pelvis, fetopelvic disproportion, or cord around the neck can contribute to face presentation. Face presentation is more common in large infants weighing more than 4000 g (42%), small infants weighing less than 2300 g (16%), and cephalopelvic disproportion. Low birth weight and cephalopelvic disproportion have also been proposed as etiologic factors in brow presentation, with associated cephalopelvic disproportion attributed to the presenting diameters of the fetal head being greater in brow presentation than in face or vertex presentations. High parity is a debated factor associated with face and brow presentations in some studies but not others. There may actually be selection for mothers with a smaller pelvis because the fetus within a larger pelvis may convert to face or vertex before being recognized as a brow presentation. About half to two thirds of all brow presentations spontaneously convert to either face (30%) or vertex presentations (20%) if given an adequate trial of labor.

When a brow converts to a face presentation, the occiput usually lodges in the maternal sacrum, and additional force on the head converts it to mentum anterior. In converting from brow to vertex, the brow engages transversely and rotates to face the pubic bones, and then the head flexes to become occipitoposterior; thus many occipitoposterior presentations may have entered the pelvis as brow presentations. In the era before prenatal diagnosis, the fetal mortality for face presentations was around 10%, with many deaths attributed to either attempted version, extraction, or conversion maneuvers (which are contraindicated with face or brow presentations) or to anencephaly. When corrected to exclude anomalous infants, the mortality rate for face presentations delivered spontaneously or by low forceps is less than 2%. Among all the proposing factors previously listed, high parity appears to be the most important. Because increased extensor muscle tone cannot cause extension of a fetal head that is fixed in the pelvis, face presentation in a primigravida woman with an engaged presenting part is less likely to occur during the last 2 to 3 weeks of pregnancy. On the other hand, the fetal head is often not engaged prior to the onset of labor in multiparous women, such that face presentation occurs more frequently.

FEATURES