Cephalopelvic Disproportion (CPD) is a diagnosis related to mechanical difficulty during childbirth. It describes a situation where the physical dimensions of the baby and the mother’s pelvis are incompatible for a safe vaginal delivery. CPD is a leading cause of obstructed labor, where the baby’s descent through the birth canal is stalled despite strong contractions.
Defining Cephalopelvic Disproportion (CPD)
CPD is fundamentally a mechanical mismatch between the fetus and the maternal pelvis. The term “cephalo” refers to the baby’s head, and “pelvic” refers to the mother’s bony birth canal, indicating a size difference that prevents passage. This mismatch occurs when the baby’s head is too large, the mother’s pelvis is too small, or a combination of both factors is present.
A diagnosis of CPD is often considered functional rather than purely anatomical. While imaging techniques can measure the size of the pelvis and the baby’s head, the final determination depends on how labor progresses. The baby’s head is designed to mold, and the mother’s pelvic joints can spread during labor, often allowing for a successful vaginal birth even when measurements suggest a tight fit. True CPD is confirmed only when the baby fails to descend through the pelvis despite effective uterine contractions.
Factors Contributing to CPD Risk
The likelihood of CPD is influenced by factors related to both the mother’s anatomy and the baby’s characteristics. Maternal factors frequently involve the size and shape of the pelvis, which may be genetically small or abnormally formed. Women of short stature, often defined as being under 145 or 155 centimeters, have a higher risk for a smaller pelvic diameter. Past trauma, such as a pelvic fracture, or conditions like rickets can also distort the pelvic structure, narrowing the birth canal.
Fetal factors primarily center on the baby’s size and positioning. Fetal macrosomia, defined as a birth weight greater than 4,000 grams (8 pounds, 13 ounces), is a significant contributor to CPD risk. Conditions like maternal or gestational diabetes can lead to larger babies with increased body fat and wider shoulders. Additionally, an abnormal fetal position, such as a face, brow, or transverse lie presentation, can create a functional disproportion even if the baby is not excessively large.
Diagnostic Methods and Confirmation
Prenatal assessment for CPD begins with trying to predict the size of the fetus and the dimensions of the mother’s pelvis, though these methods have limitations. Ultrasound is commonly used to estimate fetal weight and head circumference, but these estimations are not always accurate predictors of labor outcomes. Clinical pelvimetry, which involves a healthcare provider manually assessing the size of the birth canal, can provide an estimate of the pelvic capacity. Imaging techniques like MRI pelvimetry offer more detailed measurements of the bony pelvis, but a definitive diagnosis of CPD is rarely made before labor begins.
The definitive confirmation of CPD most often occurs during active labor. When a mother experiences a “failure to progress” despite adequate labor support and strong contractions, CPD is suspected. Failure to progress is defined by a stalled cervical dilation or the baby’s head maintaining a high station without descending into the pelvis. In such cases, a “trial of labor” allows medical professionals to observe if the natural forces of labor and pelvic flexibility can overcome the suspected size difference. If the baby’s descent remains obstructed, the diagnosis of CPD is confirmed, indicating that a vaginal delivery is unlikely to be successful.
Delivery Management and Outcomes
When CPD is confirmed during a trial of labor, the safest and most common management is an immediate Cesarean section (C-section). Continuing a prolonged, obstructed labor poses serious risks to both the mother and the baby. These risks include fetal distress due to oxygen deprivation, uterine rupture, or severe birth injuries like shoulder dystocia. The decision to proceed with a C-section prevents these complications and ensures a safe delivery.
For mothers who receive an antenatal diagnosis of a severely small or abnormally shaped pelvis, a planned C-section may be scheduled to avoid the risks of a trial of labor entirely. While a CPD diagnosis indicates a mechanical difficulty, it does not guarantee recurrence in future pregnancies. Studies suggest that a significant percentage of women who received a CPD diagnosis in a previous birth go on to have a successful vaginal delivery in subsequent pregnancies.