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Mind The Gap

By Kelly Winder

CPD is the most common reason for performing a caesarean. But how common is it, really?

In the 18th and 19th centuries, poor nutrition, rickets, and illnesses such as polio caused pelvic anomalies, which resulted in loss of life during childbirth. Indeed, initially CPD—short for cephalopelvic disproportion, a condition where the baby’s head is thought to be too large to pass through the woman’s pelvis—was the most common reason for carrying out a caesarean. In modern times, however, CPD is rare, since our general standard of living is so much higher. True CPD is more likely to be caused by pelvic fracture due to traffic accidents or congenital abnormalities.

Often CPD is implied rather than diagnosed. In cases where labor has failed to progress or the baby has become distressed, medical staff commonly assume that this is due to physical inadequacies in the mother rather than look toward circumstances of the mother’s care. But these problems frequently occur when CPD is not suspected, and they have many other causes, such as fear and uncertainty, difficulty adjusting to a medical environment, lack of emotional support and non-continuity of caregiver.

Many women worry about how something as big as a baby will come down such a narrow vaginal passage, so implications of pelvic inadequacy can confirm personal fears, lower self-esteem, affect the progress of any subsequent labor and add greatly to feelings of failure.

CPD is also sometimes suspected when the baby’s head fails to engage, although both this and failure to progress have proven unreliable indicators.

When CPD is suspected, x-ray pelvimetry may be suggested. This is when the mother’s pelvis is measured by taking x-rays to assess pelvic adequacy. Quite apart from the health risks of x-rays, this method of pelvic assessment has been criticized since it has been shown to be inaccurate, and because often the results do not influence the way that the delivery is managed. Due to concerns over x-ray exposure of women and babies, some hospitals offer pelvimetry by a computed tomography (CT) scan, which uses a much lower dose of radiation. However, there is no reason to believe that the resulting measurements provide a more accurate diagnosis of CPD than conventional x-rays, for the same reasons. What if your ultrasound says you have a big baby? Well, maybe, maybe not. The Australasian Society for Ultrasound in Medicine states the following in its policy, “Statement on Normal Ultrasonic Fetal Measurements”: “No formula for estimating fetal weight has achieved an accuracy which enables us to recommend its use.” A woman’s degree of motivation to achieve a vaginal delivery, along with the level of support she receives, are likely to be more influential on the outcome than her pelvic measurements. Even in undisputed cases of CPD, it should still be possible for a mother to go into labor without compromising her baby’s safety. In fact, a period of labor prior to caesarean section is believed to reduce the occurrence of respiratory distress, and can therefore be beneficial for the baby. In any case, CPD is difficult to diagnose accurately, since there are no less than four variables that cannot be measured:

  1. The pelvic girdle is not a fixed, solid structure. During pregnancy and labor the hormone relaxin softens the ligaments that join the pelvic bones, allowing the pelvis to give and “stretch.” The degree of pelvic expansion achieved will vary from woman to woman and from pregnancy to pregnancy.

  2. Babies’ heads mold into shape. Babies’ heads are made up of separate bones which move relative to each other, allowing the baby’s head to “mold” and thus reduce its diameter during passage down the birth canal. No one can predict the capacity of an individual baby’s head to mold. As this is a feature of the normal birth process, it should not adversely affect the health and well-being of the baby.

  3. The position that a woman adopts during labor and delivery makes a difference to pelvic dimensions. Squatting, for example, can increase pelvic measurements by up to 30 percent. One of the most common birthing positions—semi-reclined, with the mother’s weight on her coccyx—restricts movement of the coccyx, however. This can severely compromise a belowaverage pelvis.

  4. Baby’s position is important. The position of the baby can be crucial, and whether its head is well flexed or tilted can mean the difference between an easy delivery and delivery being impossible.

Prior Diagnoses of CPD

When a diagnosis of CPD has been made, many people still believe that this constitutes a reason for elective repeat caesarean section in future pregnancies, despite the wealth of evidence to the contrary. Indeed, there have been many documented cases where women have been diagnosed as having CPD and then gone on to deliver vaginally a larger infant than the one that was delivered surgically.

Karen, a birthing mom whose first baby remained high and was caesarean born due to failure to progress in labor, was diagnosed as having CPD following a CT scan. She went on to deliver her second child, a healthy 9-pound 7-ounce baby, vaginally. The likelihood of vaginal birth is not significantly altered by the indication for the first caesarean section (including “cephalopelvic disproportion” or “failure to progress”).

Some women will be able to accept and concur with a diagnosis of CPD, perhaps even preferring the caesarean way of birth. Others will want to be able to come to their own independent conclusions, and some of these may wish to labor again under more conducive circumstances, to have the chance to give labor their “best shot.”

NOTE: Parts of this article reproduced with permission from

Concerned? Here are some tips.

  1. Read about optimal fetal positioning. I have heard from many doulas and midwives (and have seen for myself) that women get sent off for caesareans due to CPD or “failure to progress” when the baby was simply in a posterior position or not in an optimal position. It is believed that modern lifestyle (more sedentary than it once was) could be a reason for babies in posterior or other less optimal presentations.

  2. Get a second opinion. If you’re being told your body is not able to give birth vaginally and you’re not happy or convinced, it’s definitely worth seeking a second opinion. Don’t give in to pressure if your caregiver isn’t willing to listen to your concerns or give you the chance to birth vaginally.

  3. Hire a private midwife or doula. By hiring your own private midwife or doula, you will have someone to listen to your needs and concerns and advocate for you, while offering some huge benefits. For example, doula-attended births result in 50 percent fewer caesarean sections (which can be more likely to happen if your ob-gyn “thinks” you have a big baby). Doula-attended births also tend to feature shorter labors, with less pain relief needed.

  4. Attend independent birth education classes. It’s a great idea to attend birth classes unaffiliated with your hospital. They offer a good range of information specific to giving birth as actively as possible. Hospital classes are often limited and are more specific to basics and pain-relief options. Private classes are not bound by policies and protocols and offer the best education for a couple wanting honest and accurate information.