Small Pelvis? Here’s The Truth About Cephalopelvic Disproportion (CPD)



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A diagnosis of CPD (cephalopelvic disproportion) is where the baby’s head is thought to be too large to pass through the woman’s pelvis.

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 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 and true CPD is more likely to be caused by pelvic fracture due to road traffic accidents or congenital abnormalities.

Often CPD is implied rather than diagnosed. In cases where labour 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 towards circumstances of the mother’s care. These problems frequently occur when CPD is not suspected and there are many other causes such as fear and uncertainty, difficulty adjusting to a medical environment, lack of emotional support and non-continuity of carer.



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 labour 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 proved unreliable indicators.

When CPD is suspected, x-ray pelvimetry may be suggested, either ante-natally or post-natally. 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 criticised since it has been shown to be inaccurate and because often the results do not influence the way that the delivery is managed.^5^ Due to concerns over x-ray exposure of women and babies, some hospitals offer pelvimetry by computed tomography (CT) scan which uses a much lower dose of radiation. However, there is no reason to believe that the resulting measurements will provide a more accurate diagnosis of CPD than conventional x-rays for the same reasons.

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 labour without compromising the safety of her baby. In fact, a period of labour 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 labour 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 mould into shape



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

3. The position that a woman adopts during labour and delivery makes a difference to pelvic dimensions

Squatting, for example, can increase pelvic measurements by up to 30%. One of the most common positions in which women give birth, that of being semi-reclined where the mother’s weight is on her coccyx, restricts movement of the coccyx, which can severely compromise a below-average pelvis.

4. Baby’s position

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.

What if I have had a previous diagnosis 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, whose first baby remained high and was caesarean born due to failure to progress in labour, was diagnosed as having CPD following a CT scan. She went on to deliver a healthy 9lb 7oz 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, whereas others will want to be able to come to their own independent conclusions, and some of these may wish to labour again under more conducive circumstances, to have the chance to give labour their ‘best shot’.



The above information on cephalopelvic disproportion (CPD) was excerpted from our article, Caesarean Myths Exploded.

Stories & 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?”

Some Tips If You Are Worried

1. Read about optimal fetal positioning

BellyBelly has an article HERE or check out the fabulous site, Spinning Babies – which all pregnant women should read. I have heard from many doulas and midwives (as well as having seen it 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, as one of the midwives commented above. 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 are not happy or convinced being told your body is not able to give birth vaginally then it’s definitely worth seeking a second opinion. Don’t give in to pressure or a carer who is not willing to listen to your concerns or give you the change 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 they will advocate for or with you, while offering some huge benefits, for example 50% less caesarean sections (which can be more likely to happen if your Ob ‘thinks’ you have a big baby) as well as shorter labours, less pain relief needed etc. You can locate a private midwife by contacting the Maternity Coalition and you can find a doula in BellyBelly’s Directory.

4. Attend independent birth education classes

It’s a great idea to attend birth classes which are external to hospital classes and 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. Check out our article 9 Reasons Why You Should Choose Independent Birth Education.

Watch me!

A great clip on YouTube from ICAN about CPD here.

Also, you can view a link in our forums which contains images (nudity) of birth – small women and large babies – you’ll be amazed at what women are capable of.

Ultrasound Says You Have a Big Baby?

Then did you know 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.”

Some Studies On Pelvimetry and CPD

1. Impey L. and O’Herlihy C. First delivery after caesarean delivery for strictly defined cephalopelvic disproportion. Obstet Gynecol 1998;92:799-803.

68% delivered vaginally in the next pregnancy, 47% with a larger baby. Of 15 women previously delivered by caesarean at full dilatation 11 (73%) delivered vaginally. In 19 patients pelvimetry had been performed. In 11 (63%) dimensions were judged to be abnormal. All underwent trial of labour and 6 (55% – including two with larger babies) delivered vaginally.

2. Phelan et al. Vaginal birth after cesarean. AMJOG 1987;157:1510-5.

“Previous indication for cesarean birth bears only little relationship to the subsequent successful vaginal delivery”.

75% of women with previous cesarean for CPD/failure to progress delivered vaginally.

3. Jongen VHWM et al. “Vaginal delivery after previous caesarean section for failure of the second stage of labour”. BJOG 1998;105:1079-81.

82 (80%) of 103 women with previous delay in descent in second stage delivered vaginally, including 41 (75%) of 55 who had a history of failed instrumental delivery.

4. Flamm BL and Goings JR. “Vaginal birth after caesarean section: Is suspected fetal macrosomia (large for dates baby) a contra-indication.”

4000-4499g range, 139 of 240 patients (58%) delivered vaginally. Greater than 4500g, 43% delivered vaginally. Comparison with control group of 301 women with no previous uterine surgery and macrosomia, showed no significant difference in perinatal or maternal morbidity.

*Parts of this article reproduced with permission from the "Caesarean.org.uk ":http://www.caesarean.org.uk/articles/Myths.html website.

Also read our article, Pelvises I Have Known and Loved by Gloria Lemay.

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