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Cephalo-pelvic Disproportion (CPD)

Cephalo-pelvic disproportion (CPD) is a term used to describe a situation where a baby's head is not able to pass through the mother's pelvis.

Most women have a pelvis that is more than adequate for giving birth naturally. Refer to the article, The Female Pelvis, Designed for Giving Birth.

However, many women are told they will not be able to give birth vaginally as the baby is "too big".

The Female Pelvis

The Female Pelvis

The true female pelvis has a circular-shaped brim. However, there are 3 other pelvic classifications:

  • the male-shaped pelvis has a heart-shaped brim
  • the oval-shaped pelvis is very narrow from side to side
  • the flat pelvis has a kidney-shaped brim and is very narrow from front to back

In rare instances, a baby can genuinely not pass through a woman's pelvic outlet.

However, CPD may only be diagnosed by a trial of labor.

Cephalo-pelvic disproportion can rarely be diagnosed before labor begins, even if the baby is thought to be large or the mother’s pelvis is known to be small. Even ultrasound estimates are unreliable.

The Australasian Society for Ultrasound in Medicine states that “No formula for estimating fetal weight has achieved an accuracy which enables us to recommend its use.1

CPD is difficult to diagnose accurately as there are a number of inter-related factors to consider, none of which can be predicted or accurately 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, enabling the pelvis to be flexible. The degree of flexibility varies from woman to woman and one labor to the next, so it can not be predicted.

2. Moulding of the baby's head

The bones of the baby's skull are still separate and are able to move relative to each other, allowing the baby’s head to reduce its diameter during the passage down the birth canal. The degree of moulding cannot be predicted. This is a normal process and should not affect the well-being of the baby.

3. Mother's positioning for labor and birth

The positions that a woman adopts during labor and delivery may facilitate the maximization of the size of the pelvic outlet. Upright or squatting positions, rather than back-lying positions will be beneficial as they enable pelvic measurements to increase.3

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. Babies are able to and do change position.

In a case where there is failure to progress during labor and all attempts have been made to facilitate vaginal birth, a baby may need to be born by cesarean.

CPD occurs in 1 out of 250 pregnancies3 and 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.

In another study, 68% delivered vaginally in the next pregnancy, 47% with a larger baby.4

Genuine cases of cephalo-pelvic disproportion are rare, yet it is vastly over-diagnosed.

Click here to view a Vaginal Childbirth Video showing women who experienced vaginal births after being told their babies were too big!


1. Australasian Society for Ultrasound in Medicine, Statement On Normal Ultrasonic Fetal Measurements, Aust N Z J Obstet Gynaecol. 2002 Feb;42(1):101-3.

2. Gupta J, Glanville J, Johnson N, et al. The effect of squatting on pelvic dimensions. Eur J Obstet Gynecol Reprod Biol 1991;42: 19-22.

3. American College of Nurse Midwives(ACNM),

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

More Pages Related to the Cephalo-pelvic Disproportion

The Female Pelvis, Designed for Giving Birth

Use a Birth Ball to Maximize the Pelvic Outlet

Big Baby Birth Story - vaginal birth of a 16 pound baby

VBAC - Vaginal Birth After Cesarean


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