Cephalopelvic disproportion (CPD) occurs during childbirth when the baby’s head or body cannot pass through the mother’s pelvis, resulting in obstructed labor. This mismatch between the fetal size and the maternal pelvic capacity prevents the natural progression of delivery. CPD is a relatively rare complication, occurring in about 1 out of every 250 births. When CPD stalls the birthing process, medical intervention is required to ensure the safety of both the mother and the baby. The inability of the fetus to descend despite strong uterine contractions is the defining sign that labor is not progressing.
Defining Cephalopelvic Disproportion
Cephalopelvic disproportion is a physical incompatibility between the capacity of the mother’s pelvis and the size or presentation of the baby. While the pelvis is designed to expand somewhat during labor, a true anatomical mismatch cannot be overcome.
The maternal pelvis is divided into the false pelvis and the true pelvis, which is the bony canal the baby must pass through. The shape of the true pelvis dictates the ease of passage. Four types are recognized, with the gynecoid shape being the most favorable for birth. Shapes like the android (heart-shaped) and platypelloid (flat, oval) are associated with a higher risk of CPD because their diameters are narrower.
CPD is primarily a functional diagnosis, determined when labor fails to progress. Although prenatal measurements may suggest a possible size issue, the diagnosis is confirmed only when the baby’s head fails to descend during active labor. “True CPD” refers to an absolute size mismatch, but often, the disproportion is functional, stemming from a less-than-ideal fetal position that temporarily prevents passage.
Identifying the Causes and Risk Factors
The causes of CPD are grouped into factors related to the mother’s anatomy and factors related to the baby’s size or positioning. Understanding these risk factors allows healthcare providers to monitor pregnancies more closely.
Maternal factors relate to the size or shape of the pelvis, influenced by genetics, previous trauma, or skeletal development. Women who experienced malnutrition during childhood, are of short stature (under 145 cm), or have a history of a fractured pelvis may have a smaller or abnormally shaped pelvic opening. Specific pelvic shapes, such as the heart-shaped android pelvis, naturally narrow the passageway.
Fetal factors primarily involve size, known as fetal macrosomia, where the baby weighs over 8 pounds, 13 ounces (4,000 grams) at birth. Maternal conditions like gestational or pre-existing diabetes can lead to macrosomia by promoting larger growth due to increased glucose availability. Delivering a baby past the due date (postmaturity) also increases the risk, as the baby continues to grow in utero.
The baby’s position is another significant factor. Even an average-sized baby may not fit if positioned poorly, such as in an occiput posterior position (where the baby faces the mother’s abdomen). An abnormal fetal head presentation can present a larger diameter to the pelvis than the optimal tucked-chin position. Additionally, conditions like hydrocephalus, which causes fluid buildup in the head, can disproportionately increase the fetal head circumference.
Diagnosis and Assessment
Diagnosing CPD is complex because it cannot be definitively confirmed before labor begins, as the maternal pelvis and the fetal head mold and adapt during the process. Pre-labor assessments can only suggest a risk of CPD, not a certainty. CPD is primarily a clinical diagnosis made during the active phase of labor.
Healthcare providers use ultrasound imaging to estimate the baby’s weight and size, but these measurements are estimates and often have a significant margin of error. Clinical pelvimetry, a manual internal examination, allows the provider to assess the dimensions and shape of the pelvis, but this method is also not always accurate in predicting labor outcome. During labor, the most telling sign of CPD is a “failure to progress.”
Failure to progress occurs when the cervix stops dilating or the baby’s head stops descending into the pelvis despite adequate and strong uterine contractions. For a first-time mother, this is considered if active labor lasts 20 hours or longer, or if no change in dilation or descent is noted over several hours. This arrest of labor, even after medical attempts to augment contractions with oxytocin, indicates a physical obstruction is present, leading to the functional diagnosis of CPD.
Management and Delivery Options
Once true CPD is diagnosed or strongly suspected due to a failure to progress during active labor, the safest management strategy is an immediate Cesarean section (C-section). Attempting a vaginal delivery against a true disproportion carries severe risks, including fetal distress, shoulder dystocia, and uterine rupture. A C-section ensures a controlled delivery, significantly reducing the chance of trauma to both the mother and the baby.
A trial of labor is often conducted, even when risk factors are present, because many cases of suspected disproportion resolve when the fetal head molds or the mother changes positions. Positional changes can facilitate the baby’s descent by optimizing the available pelvic space. If the failure to progress is due to ineffective contractions rather than a physical block, labor augmentation medications like oxytocin may be used to strengthen contractions and promote delivery.
In rare instances, such as known severe pelvic deformity or an exceptionally large estimated fetal weight, CPD may be diagnosed before labor, and an elective C-section may be planned. However, most CPD diagnoses occur acutely when labor arrests. When CPD is properly managed with a timely C-section, the long-term well-being of both the mother and the newborn is generally not affected.