Cephalopelvic disproportion is the medical term typically used to characterize the condition where the fetal head or body is too large to enter or properly pass through the maternal pelvis and birth canal.
Cephalopelvic disproportion typically occurs where either the fetal head or body is too large, or the maternal pelvis is too small, to permit the baby to be delivered via a normal spontaneous vaginal delivery.
The baby’s head or body may become too large, giving rise to cephalopelvic disproportion, when the mother suffers from poorly controlled diabetes during pregnancy. Poorly controlled diabetes during pregnancy can interfere with the normal supply of sugars and other nutrients to the baby, which, in turn, can result in abnormal growth of the fetal head and body. Other conditions that may contribute to the fetal head or body becoming too large during pregnancy include a post-term or post-dates pregnancy and hydrocephalis.
During pregnancy, the baby’s head and body size ordinarily can be assessed by ultrasonographic imaging; most commonly, the fetal head circumference and biparietal diameter can be easily measured during a routine ultrasound examination. By comparing fetal biometric measurements against standardized growth charts, the safe and careful obstetrician can determine the relative risk of cephalopelvic disproportion by the time of delivery.
When cephalopelvic disproportion is diagnosed, proper and appropriate obstetrical management may require the delivery of the baby by cesarean section.
As stated above, cephalopelvic disproportion may occur even when the baby is of a normal size in those cases where the maternal pelvis is too small to permit a normal, spontaneous vaginal delivery. The assessment of the shape of the maternal pelvis may assist the safe and careful obstetrician in evaluating the relative risk of cephalopelvic disproportion associated with vaginal delivery.
Thus, prior to any vaginal delivery, the safe and careful obstetrician will assess the dimensions of the maternal pelvis, using a process known as pelvimetry, and will determine whether the maternal pelvis can safely accommodate the passage of the baby, when considered in light of the expected or measured size of the baby’s head and body.
During labor, a diagnosis of cephalopelvic disproportion can be made by the safe and careful obstetrician when the progress of labor arrests, or stops, or fails to follow the expected rate of descent, referred to as the “Friedman curve.”
Failure to achieve progress in cervical dilatation or descent of the baby for a period of two hours or more can herald cephalopelvic disproportion and should alert the safe and careful obstetrician to the possible need of an operative delivery of the baby, including possible cesarean delivery.
Any failure to diagnosis a cephalopelvic disproportion may result in a shoulder dystocia, where in the baby’s shoulder is impacted against the mother’s pubic bone and cannot be delivered without the use of various obstetrical maneuvers. If the shoulder dystocia is not managed properly, the baby can suffer neurologic injury, such as a brachial plexus injury.
Cephalopelvic disproportion is also associated with compression of the umbilical cord which can lead to fetal asphyxia and hypoxic-ischemic encephalopathy.