Baby Too Big For Pelvis: The Truth About Cephalopelvic Disproportion

January 2008 Newsletter, by Carol Peterson, ICCE, CD(DONA)

Have you, or someone you know, been told that a cesarean occurred due to CPD, or cephalopelvic disproportion?  Do you really know what this term means?  And, how will it affect future labors?  The true definition of CPD is when a baby's head or body is too large to fit through mom's pelvis.  It is believed that this occurrence is rare, but a large number of labors that are deemed "failure to progress" are labeled as probable CPD.  When an accurate case of CPD is found, the safest way for baby to be born is via cesarean; however, CPD is not something that can be easily determined without a trial of labor.

What causes CPD? 

There are several reasons why a labor does not progress past a certain dilation, even when adequate time is given.  Under these circumstances, it is possible for a caregiver to use the term cephalopelvic disproportion.  While the true definition is when baby is too big for the pelvis, these other factors may also lead to a diagnosis of CPD:

In the case of odd fetal position, some medical providers will give up too soon and perform a cesarean.  Many doctors follow a 1 cm dilation per hour rule, which rarely holds true for any labor.  Given time, many women are still able to give birth vaginally.  I have even seen a midwife gently reposition a baby's head once mom was fully dilated, which allowed mom to give birth vaginally.  

Does this mean that I should just have a cesarean if my baby is big?  I am very petite or have been told I have a small pelvis... shouldn't I just have a cesarean?

No!!!  Ultrasounds can be wrong, and any type of pelvic measurement or x-ray will not take into account the ability of a pelvis to spread.  Also, a person's size in no way determines her ability to birth.  Even tiny women can give birth to 10 pound babies.  I personally know of several girls that are very petite and had 9 and 10 pound babies.   While cesareans are safer today than in years past, they are still not safer than vaginal deliveries in the majority of circumstances.

How is CPD diagnosed?

The diagnosis of cephalopelvic disproportion is often used when labor does not progress after many hours.  Often medical intervention enters the labor, when Pitocin is used in an attempt to cause labor to progress.  Pitocin may or may not cause dilation, and in a case of CPD the baby will often still remain high in the pelvis even if dilation occurs, and pushing will not bring him further down.  Often, in a case of true CPD, a baby's head will show no molding, which is a sign that the baby was not able to enter the pelvis.  

CPD can rarely be diagnosed before labor begins even if the baby is thought to be large or the mom’s pelvis is known to be small. During labor, the baby’s head molds and the pelvis joints spread, creating more room for the baby to pass through the pelvis. Ultrasounds are used to estimate fetal size, however they are not 100% accurate in determining weight. While a physical examination is often used to determine pelvic size, a 2002 study showed that it is difficult to determine a woman's ability to birth without a trial of labor, even if a woman has been identified as having an odd-shaped pelvis, and even when MRIs and X-Rays are used. (1)   

In many cases a diagnosis of CPD is made when there is no other explanation of why dilation does not occur, or when labor does not follow the 1 cm dilation per hour guideline.

What about future pregnancies?

Cephalopelvic disproportion is a rare occurrence. According to the American College of Nurse Midwives(ACNM), CPD occurs in 1 out of 250 pregnancies. If you have been diagnosed with CPD, this does not automatically mean that you will have this problem in future deliveries. According to a study published by the American Journal of Public Health, over 65 % of women who had been diagnosed with CPD in previous pregnancies were able to deliver vaginally in subsequent pregnancies.

If you were given a diagnosis of CPD with a prior labor, be aware that there are many causes, and it may have nothing to do with your pelvis!  Perhaps your baby was in an odd position, or perhaps you were given up on too soon.  Check out the video below for inspiration!  (In the video you will see acronyms:  CPD: cephalopelvic disproportion; VBAC: vaginal birth after cesarean; HBAC: Hypnobirth after cesarean; UBAC: underwater birth after cesarean.)  

Information for this article was taken from Cephalopelvic Disproportion, published by the American Pregnancy Association, www.americanpregnancy.org. 

(1) MR Imaging Pelvimetry: A Useful Adjust in the Treatment of Women at Risk for Dystocia?, AJR 2002, 179:137-144