Brow presentation is a rare fetal position during childbirth where the baby’s head is partially extended, causing the forehead to lead the way through the birth canal. This differs from the common head-first (vertex) presentation, where the chin is tucked to the chest. When the brow presents, it can significantly affect the birthing process, often requiring medical intervention.
Understanding Brow Presentation
In a brow presentation, the fetal head is positioned midway between full flexion (as in a vertex presentation) and complete hyperextension (as in a face presentation). The part of the fetal head that first enters the maternal pelvis is the area between the orbital ridge and the anterior fontanel, which includes the forehead. This contrasts with a vertex presentation where the top of the head presents, or a face presentation where the face and chin lead.
The challenge with a brow presentation lies in the diameter of the fetal head that attempts to pass through the birth canal. In this position, the largest diameter of the fetal head, known as the occipitomental diameter, is the presenting part. This diameter measures approximately 13.5 cm, which is considerably larger than the typical presenting diameter in a vertex presentation (around 9.5 cm). Such a large presenting diameter often prevents the head from engaging properly in the maternal pelvis, making vaginal delivery difficult or impossible.
This malposition is unstable, as it may spontaneously convert to a more favorable vertex (flexed head) or face (hyperextended head) presentation as labor progresses. If it persists, the inability of the head to descend can lead to prolonged labor and an increased risk of complications. This presentation is rare, occurring in about 1 in 500 to 1 in 4000 deliveries.
Identifying Brow Presentation
Healthcare professionals typically diagnose brow presentation during labor, often through a vaginal examination. While abdominal palpation (Leopold maneuvers) might occasionally suggest a malposition, it is generally unreliable for confirming a brow presentation. During a vaginal examination, the practitioner can feel specific fetal landmarks that distinguish a brow presentation from other positions.
Key indicators during a vaginal examination include the palpation of the large anterior fontanelle, the frontal sutures, the orbital ridges, the eyes, and the root of the nose. Crucially, in a brow presentation, the chin and mouth cannot be felt, which helps differentiate it from a face presentation. The absence of the sagittal suture and posterior fontanelle further distinguishes it from a vertex presentation.
Ultrasound evaluation provides definitive confirmation of brow presentation, showing a hyperextended fetal neck. This imaging method is useful for objective assessment of the fetal head’s attitude when a malpresentation is suspected. Diagnosis is often made in the second stage of labor.
Factors Contributing to Brow Presentation
Brow presentation can arise from various maternal and fetal factors, though sometimes no specific cause is identified. Maternal factors include abnormalities in pelvic shape, such as a contracted or platypelloid pelvis, which can impede the proper descent and flexion of the fetal head. A lax uterus, often seen in individuals who have had multiple previous pregnancies (multiparity), might also contribute by not holding the baby firmly in a stable position. Previous cesarean delivery is another maternal risk factor.
Fetal factors also play a role in the development of a brow presentation. These can include conditions that affect the fetal head or neck, preventing proper flexion. Examples include fetal anomalies such as anencephaly, hydrocephalus, or neck masses that physically obstruct head flexion. Macrosomia (a large baby) or, conversely, prematurity and low birth weight, can also be associated with this presentation. Additionally, an excess of amniotic fluid (polyhydramnios) might provide too much space, making it difficult for the baby to settle into an optimal position.
Navigating Delivery with Brow Presentation
Once identified, healthcare providers assess the situation to determine the safest course of action. A significant proportion of brow presentations (more than 50%) are transient and will spontaneously convert to either a vertex or face presentation during labor. Expectant management, involving close monitoring, may be an initial approach if labor is progressing and the baby’s well-being is stable.
Persistent brow presentation, where the head does not convert to a more favorable position, generally has a poor prognosis for vaginal delivery. This is due to the large occipitomental diameter of the fetal head, which is too wide to pass through the maternal pelvis. In such cases, a cesarean section is typically the recommended and safest mode of delivery to avoid complications for both the birthing parent and the baby.
Vaginal delivery of a persistent brow presentation is generally considered impossible, except in rare instances where the fetus is very small or the maternal pelvis is exceptionally large. Attempts at manual conversion to a vertex or face presentation are rarely successful and carry significant risks, including uterine rupture and fetal injury. Such maneuvers are generally not recommended.
Complications associated with persistent brow presentation and attempted vaginal delivery include prolonged labor, fetal distress, and an increased risk of injury to the birthing parent and baby. Babies born after a brow presentation may experience significant molding of the head or swelling of the face, though these typically resolve. Continuous fetal heart rate monitoring is important throughout labor to detect any signs of fetal compromise.