While most babies present head-first with their chin tucked to their chest, known as vertex presentation, variations can occur. One such variation is face presentation, which happens in approximately 1 in 600 to 800 live births, or about 0.1% to 0.2% of deliveries. This position can complicate the birthing process, and understanding its underlying causes helps in managing deliveries effectively.
Defining Face Presentation
Face presentation is a cephalic malpresentation where the fetal head and neck are hyperextended, causing the back of the head (occiput) to touch the upper back of the fetus. The face, from the forehead to the chin, becomes the presenting part in the birth canal. In contrast, the more common vertex presentation involves the baby’s head being flexed, with the chin tucked tightly against the chest, allowing the crown of the head to lead the way. During a face presentation, the fetal chin (mentum) serves as the reference point for healthcare providers.
Fetal Factors Leading to Face Presentation
Fetal characteristics and conditions contribute to face presentation. The primary mechanism involves the baby’s head extending backward, rather than flexing forward, as it descends towards the pelvis. This hyperextension can be influenced by various fetal anomalies, such as anencephaly, a condition where a significant portion of the brain and skull is absent, often associated with face presentation. Other anomalies like neck masses or tumors, including fetal thyromegaly, can also physically prevent the head from flexing normally.
Fetal size also plays a role. A baby that is significantly larger than average, a condition known as macrosomia, may struggle to flex its head adequately within the confines of the uterine and pelvic spaces, leading to an extended position. Conversely, premature babies, particularly those with very low birth weight, might also be more prone to unusual presentations due to their smaller size and less developed muscle tone, which can affect their ability to maintain a flexed posture. In pregnancies involving multiple fetuses, such as twins, the restricted space within the uterus can limit fetal movement and predispose one or more babies to face presentation.
Maternal and Uterine Contributions
The mother’s anatomy and uterine condition can influence whether a baby presents face first. An unusually shaped or contracted pelvis, a condition sometimes referred to as cephalopelvic disproportion, can prevent the baby’s head from flexing properly as it attempts to descend. For example, a flat pelvis might allow the head to engage in a way that promotes extension.
Uterine abnormalities, such as the presence of fibroids or a bicornuate uterus, can alter the normal shape of the uterine cavity. These structural variations can restrict the baby’s ability to move freely and position itself optimally, thereby increasing the likelihood of a face presentation. Additionally, women who have had multiple previous births, known as multiparous women, may have looser abdominal and uterine muscles. This reduced uterine tone can provide the fetus with more room and mobility, potentially allowing for less stable positions and increasing the chances of a face presentation.
Other Contributing Influences
Other elements can also influence the baby’s position during birth. Abnormalities in the volume of amniotic fluid, either too much (polyhydramnios) or too little (oligohydramnios), can affect fetal mobility and positioning. Polyhydramnios provides the baby with greater space to move, potentially allowing it to adopt an extended, face-presenting position. Conversely, oligohydramnios can restrict movement, making it difficult for the fetus to adjust to a more favorable presentation.
The location of the placenta can also play a role. Placenta previa, a condition where the placenta partially or completely covers the internal opening of the cervix, can obstruct the lower uterine segment. This obstruction can prevent the fetal head from properly engaging in the pelvis and may contribute to an extended position. Issues with the umbilical cord, such as a short cord or multiple nuchal cords (cord wrapped around the baby’s neck), can restrict the baby’s ability to flex its head. These cord issues can physically impede movement and contribute to the head extending backward, leading to a face presentation.