Brow presentation is a rare malpresentation where the fetal head occupies a position midway between complete flexion (vertex) and complete extension (face). The presenting part is the area of the fetal head between the orbital ridge and the anterior fontanelle, known as the brow or mentoccipital diameter. Because this diameter measures approximately 13.5 centimeters, it is the longest anteroposterior diameter of the fetal skull. Passing this wide measurement through the normal maternal pelvis is exceptionally difficult, making spontaneous vaginal delivery nearly impossible in a persistent brow presentation.
Most cases are unstable during early labour. The presentation often starts as an intermediate attitude of the head and may spontaneously convert either into a favourable vertex presentation with further flexion or into a face presentation with further extension. Midwives must understand that a persistent brow presentation represents mechanical dystocia, requiring careful evaluation in a well-equipped maternity unit.
Any condition that prevents normal flexion or encourages extension of the fetal head can result in a brow presentation. These factors may be maternal, fetal, or placental in origin.
Discovering a brow presentation early prevents prolonged obstructed labour. Midwives rely on abdominal and vaginal examinations during labour to identify the condition.
Abdominal palpation often reveals an unusually high head that feels very prominent on both sides. The occiput is easily felt at a higher level than the chin, and a deep depression or groove may be palpable between the occiput and the back.
Vaginal examination is definitive. The midwife feels the large anterior fontanelle, the orbital ridges, the root of the nose, and the eyes. The mouth and chin are out of reach, which distinguishes it from a face presentation. Extensive caput succedaneum and marked moulding frequently develop over the brow area due to the tight fit, making identification via palpation confusing if the examination is delayed.
When a brow presentation persists, the course of labour becomes abnormally prolonged. Engagement rarely occurs because the mentoccipital diameter is too wide for the pelvic inlet. If the pelvis is unusually roomy and the fetus is very small, a rare mechanism of labour may happen.
The brow descends with the frontal bone acting as the leading part. As it hits the pelvic floor, internal rotation of the brow occurs to bring it anteriorly under the pubic arch. A degree of flexion and extension then allows the head to sweep over the perineum, but this requires extreme stretching of maternal tissues and severe moulding of the fetal head. In the vast majority of cases, progress stops entirely, resulting in secondary arrest of dilation and deep transverse arrest.
Active management is crucial upon diagnosing a persistent brow presentation to protect both mother and baby from severe trauma. Midwives must recognize their limitations and arrange prompt referral to a specialist obstetric unit.
The mechanical strain of a brow presentation poses serious risks to maternal and perinatal well-being. Prolonged labour and obstructed passages increase the likelihood of severe outcomes.
For the mother, the main hazards include uterine rupture from prolonged contractions against an unyielding obstruction, postpartum haemorrhage due to uterine atony, and extensive soft tissue trauma including vesicovaginal fistulae from prolonged pressure necrosis.
For the baby, perinatal mortality and morbidity are significantly elevated. Fetal distress and hypoxia arise from prolonged cord compression and reduced placental perfusion. Severe caput succedaneum, extensive facial bruising, and intracranial haemorrhage can also occur due to the intense pressure exerted by the bony pelvis during attempted descent.