Small Pelvis? Big Baby? Here’s The Truth About CPD

“You're really small. Are you planning a c-section?”

“Your baby bump is huge! Looks like a big baby is on the way. Are you sure he'll fit?”

If you're on the petite side, or if you're expecting a larger baby or if you're a first time mother-to-be, you might hear a great deal about babies being too big to fit through the pelvis or birth canal.

When a baby cannot fit through the pelvis, this is called cephalopelvic disproportion (or CPD for short). While we might hear this term every now and again, true cases of CPD are actually pretty rare.

In the 18^th and 19^th century, poor nutrition that resulted in rickets, and illnesses such as polio, lead to pelvic anomalies. These pelvic anomalies made CPD a somewhat common occurrence — one that resulted in loss of life during childbirth. As our nutrition and lifestyle have improved, pelvic anomalies have become rare.

Today, true cases of CPD are few and far between. In nearly every birth, the pelvis loosens and moves to make room for baby.

In the few cases where CPD does occur, it's often the result of congenital abnormalities or severe injuries, for example, a pelvic fracture during a traffic accident.

If CPD Is Rare, Why Do We Hear About It So Often?

Unfortunately, while our lifestyle helped to reduce the rate of pelvic anomalies, our birth culture shifted to non-evidenced based practices.

These non-evidenced based practices led to many implied cases of CPD making it seem like a common concern. These implied, not truly diagnosed cases, are usually the result of other things like failure to progress, which is often simply failure to wait.

If you're concerned about CPD, know that research has shown that it's nearly impossible to diagnose CPD before labour has been well established. Unless you have a pelvic injury or congenital abnormality, it isn't even easy to predict the risk of CPD before labour.

If there are sound concerns about CPD, waiting for spontaneous labour isn't generally contradicted. In this case, you can go into labour naturally, and mother and baby can benefit from knowing baby is ready to be born.

With everything we hear about babies “not fitting”, it can be hard not to be concerned.

Here are 3 things you should know about birth and CPD:

#1: Neither Your Pelvis Or Your Baby's Head Is A Fixed Object

The pelvis is not one solid bone. It is made up of several bones held together by ligaments. During pregnancy your body releases the hormone relaxin. Its release causes your ligaments and joints to loosen to facilitate baby's movement through the birth canal. This hormone is why you might experience joint weakness and discomfort towards the end of pregnancy.

Baby's skull is made up of separate bones that allow their head to mould and fit through the birth canal. These separate, not yet fused, bones is why babies have ‘soft spots' known as fontanels.

#2: Your Position Makes A Big Difference

Being on your back or being in a semi reclined position during birth can narrow pelvic measurements by 30%! When you're giving birth, 30% can make quite the difference. Squatting, side lying or being on all fours can create optimal space for baby to descend.

#3: Baby's Position Is Important

Babies are designed to descend and navigate through the birth canal. They are an active participant in birth, moving and flexing through your pelvis. Occasionally babies are not in an optimal position making labor difficult, and sometimes making it impossible for baby to navigate the birth canal. In many cases, different things can be done to encourage optimal fetal positioning to facilitate a vaginal delivery. If baby simply can't get into a proper position, a c-section might be necessary. In this case, a proper diagnosis of mal-positioning and not CPD can help mamas plan for a future VBAC.

Does A Previous Diagnosis Of CPD Mean Repeat C-Section?

Many cases of CPD are actually implied and not properly diagnosed. Reviewing your medical records and discussing them with a healthcare provider can help you understand why CPD was suspected.

In the absence of pelvic abnormalities, being that the pelvis isn't a single fixed bone, it can have varying measurements from one birth to another.

Knowing the position of the baby can also help in understanding why labor didn't progress. Did baby have any bruising or molding that indicated their head was tilted or flexed? What position were you in while laboring and pushing?

Stories Of CPD Experiences From Australian Midwives:

“I met a woman who told me she had had a caesarean section for CPD with 1st bub. Had a homebirth with 2nd, who was 2lb heavier. I believe CPD does occur, but is very rare.”

“Not really a story, but a lot of the cases of ‘CPD’ where I work, I notice the babies either have asymmetrical moulding, or bruising that is not right where it should be. Most of these babies are asynclitic presentations, it seems, and are being labelled as CPD, scaring the women into future caesarean sections without the facts.”

“There is a dishonesty and lack of true understanding of the abilities of women’s bodies, let alone birth, by those who readily use terminology like CPD to justify forceps and vacuum deliveries, not birth, in this instance! Where is the logic that the head will only fit when pulled out?”

If you're concerned about CPD, here are some tips:

#1: Choose A Healthcare Provider You Trust

When deciding the type of healthcare provider you would like to work with, private midwife, midwife or obstetrician, ask questions. Find out how often they diagnose CPD, how they handle subsequent pregnancies after CPD and if they frequently diagnose CPD during pregnancy. Their answers can help you understand what kind of care you might expect. If they diagnose CPD often and before labor begins they might not be using evidenced based practices for diagnosing CPD.

#2: Learn About Optimal Fetal Positioning

As mentioned, baby's position is important. While most babies get into proper position without any aid, occasionally some will settle into difficult positions. Our more sedentary lifestyle (most of us no longer walk and squat frequently throughout the day) can also lead to baby being posterior which is face up, instead of anterior which is facing mama's spine. Reading the link above regarding optimal positioning is very helpful.

#3: Take An Independent Childbirth Education Class

Taking a childbirth class that builds your confidence, provides you with evidenced based information and portrays birth as the normal body process that it is, can help you prepare for a positive birth experience. When you know ways to prevent implied CPD, when you know signs of true CPD, you are better prepared to have a positive birth experience regardless of how labor unfolds. You will have the information and tools necessary to make informed decisions about your care.

#4: Hire A Doula

Having a doula improves birth outcomes. Antenatally, your doula provides you with evidenced based information and helps to build your confidence in your ability to give birth. You also have the emotional security of knowing you will have constant support. Many doulas can aid in facilitating optimal fetal positioning and encouraging your to give birth in an upright position. Both of which can help avoid a misdiagnosis of CPD.

#5: Get Another Opinion

If your provider mentions CPD during your first pregnancy, or you're planning a VBAC after suspected CPD, seek a second (or even a third, or fourth) opinion. You deserve a provider that will listen to your concerns and one that uses evidenced based practices. If you're feeling unnecessary pressure to plan for a c-section, get another opinion.

What If My Ultrasound Shows a Big Baby?

Ultrasound can be a wonderful diagnostic tool, but as with all things it has its limits. Ultrasounds are generally accurate for dating and measurements in the first trimester, but towards the end of pregnancy the accuracy is much less. It is important to note that the Australasian Society for Ultrasound in Medicine in their policy, ‘Statement On Normal Ultrasonic Fetal Measurements,’ states the following: “No formula for estimating fetal weight has achieved an accuracy which enables us to recommend its use.”

Recommended Reading

Read more about CPD and find links to CPD-related studies here.

Watch me!

A great clip on YouTube from ICAN about CPD: