Strictly speaking, cephalopelvic disproportion (CPD) is when a baby’s head is too large to fit through the mother’s pelvis. When a baby is too large, it becomes challenging if not impossible for the baby to be delivered vaginally.
The term is often applied more generally by OB/GYNs as the cause of labors that for whatever reason become obstructed and fail to progress. The safest option once CPD is diagnosed is to perform a c-section. Usually a c-section is planned in advance if doctors know that a baby is extremely large or that there is a complication with the mother, however, CPD is often discovered during labor.
With true CPD, there is a mismatch in size between the mother’s pelvis and the baby’s head. This is either due to the baby being especially large or the mother’s pelvis being especially small.
The medical term for when the fetus is overly large is fetal macrosomia. Macrosomia is defined as over 8lbs 13 oz. About 10% of pregnancies, and 50% of pregnancies with gestational diabetes, are affected by macrosomia. But how does a baby grow too big to be born vaginally? Babies can be large for a number of reasons, including:
- Physically large or obese parents (hereditary factors)
- Mothers with diabetes or gestational diabetes
- Having a male vs. a female baby
- Post-term pregnancy (baby still hasn’t been born past due date)
It is also possible for mothers to have small or abnormally shaped pelvises. This occurs because of a prior injury to the pelvis or genetic factors. Adolescents and shorter women are more likely to experience this problem. An injury or malformation of the pelvis can also affect childbirth. The pelvis may be misshapen, have bony growths, or have a bone out of place.
Other risk-factors include polyhydramnios (excess amniotic fluid) and multiparity (having given birth previously, either vaginally or by c-section).
Obstetricians can use radiologic pelvimetry, a type of imaging technology that measures the dimensions of the mother’s pelvis, to predict or confirm CPD. Ultrasounds can be used to estimate the size of the baby’s head. However, studies have found a poor correlation between the use of imaging technologies and labor outcomes.
This is because the bones in the baby’s head are meant to change shape in order to pass through the birth canal. Many petite mothers carrying babies with heads that appear large are still able to successfully deliver vaginally. Additionally, imaging technologies are not as precise as obstetricians would like. They can only estimate the baby’s size, not measure it exactly.
Given these circumstances, doctors do not necessarily schedule a c-section in advance just because a baby looks large or a mother’s pelvis seems small. A c-section may not be indicated unless it is obvious that the baby will be too big for the mother to deliver vaginally or there is some other complication with the pregnancy such as a prior pelvic injury. What this means is that CPD is often diagnosed during labor, not before.
During labor, doctors monitor the position of the baby in the birth canal, uterine contractions, and the dilation of the cervix. When doctors see that the baby is not moving through the birth canal as expected, they may administer a drug such as oxytocin to stimulate labor. If the labor is still slow or if labor becomes completely obstructed, doctors will perform an emergency c-section to deliver the baby. If they don’t promptly perform a c-section, the baby may suffer from oxygen deprivation, which can cause a number of birth injuries, including cerebral palsy.
CPD happens in about 1 out of every 250 births. In otherwise normal pregnancies, babies are macrosomic 10% of the time.
There are several factors that make pregnancies high-risk. The specific risk factors for CPD include gestational diabetes, fetal macrosomia, genetic predisposition, and post-term pregnancies.
Just because a mother experiences CPD in one pregnancy does not mean that she will experience it again in subsequent pregnancies. In a study, more than 65% of women diagnosed with CPD gave birth vaginally to another child. Usually in this situation, mothers can elect to have a c-section or attempt a vaginal delivery.
Doctors may use drugs such as oxytocin that induce labor as a first response to labors that are not progressing normally. Additionally, with conditions such as shoulder dystocia when a baby is stuck, obstetricians may use tools like forceps or vacuum extractors to reposition and guide the baby.
However, with true CPD, the baby has little chance of fitting past the mother’s pelvis. Doctors need to quickly recognize fetal distress and obstructed labor and perform a c-section in order to prevent the baby from being seriously injured. Repeated attempts to deliver vaginally that prolong labor are dangerous.Medical Literature
- “Vaginal Birth After Cesarean for Cephalopelvic Disproportion: Effect OF Birth-Weight Difference on Success” by Lorie M. Harper, M.D., et al., Obstet Gynecol., 2011.
In this study, researchers were looking to estimate the effect of birth-weight differences between current and past pregnancies in women who previously had a cesarean delivery. In second pregnancies that successfully resulted in a vaginal birth after cesarean (VBAC), they usually only needed a cesarean in the first pregnancy because of non-recurring indications like malpresentation – or, an abnormal positioning of the baby during delivery. However, if the first pregnancy required a cesarean birth because of a recurring condition like CPD, the likelihood of a successful VBAC fell significantly. Is it possible that the baby’s birth weight affects the likelihood of a failed VBAC for women in their second pregnancy that had a cesarean delivery due to CPD the first time around? The results of this study found a small impact of birthweight difference in patients whose prior cesarean was performed for failed induction and non-reassuring fetal status. The conclusion of the study was that the effect of birth weight difference is small and shouldn’t be used as a critical factor in deciding whether VBAC should be attempted. Allowing women with a larger infant than their prior pregnancy to have a VBAC doesn’t seem to increase the risk of complications.
- “The Active Management of Impending Cephalopelvic Disproportion in Nulliparous Women at Term: A Case Series” by James M. Nicholson & Lisa C. Kellar, Journal of Pregnancy, 2010.
The Active Management of Risk in Pregnancy at Term (AMOR-IPAT) protocol is associated with many different studies that involve a significant reduction in cesarean delivery rate. In every one of these studies, there were nulliparous women – i.e., women who have not given birth before – who were at risk of Cephalopelvic disproportion (CPD). Risk factors for CPD in women that are pregnant for the first time are especially important because they signify the precursors for a cesarean delivery. This study looks at three examples of nulliparous women that are exposed to the AMOR-IPAT protocol, where each woman’s risk for CPD was used to estimate her Upper Limit of Optimal Time of Vaginal Delivery for CPD (UL-OTDcpd). What does this mean? The woman’s UL-OTDcpd basically means the optimal time for a woman to go into labor and have an uncomplicated birth. It is further noted that the impact of AMOR-IPAT on nulliparous patients is so important because it affects the woman’s future pregnancies as well. Ultimately, the study shows that using the AMOR-IPAT protocol to determine the woman’s UL-OTDcpd and carefully inducing patients who haven’t begun labor by that time may be an effective way of lowering cesarean delivery rates in women during their first pregnancy.
- “Evolution of the human pelvis and obstructed labor: new explanations of an old obstetrical dilemma” by Mihaela Pavličev et al., American Journal of Obstetrics and Gynecology, 2019.
This article reviews how the process of evolution has shaped the human pelvis and how this has impacted childbirth. Humans are different than other primates when it comes to the safety of giving birth—for us it is much more dangerous. The authors discuss CPD, explaining how the dimensions of the human body can lead to obstructed labor. Interestingly, they predict a rise in the rate of CPD directly related to the evolutionary pressure caused by the rise of c-sections.