Cephalopelvic Disproportion ( CPD )

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The term 'cephalopelvic disproportion' implies disproportion between the head of the baby ('cephalus') and the mother's pelvis.

Complications can occur if the fetal head is too large to pass thorugh the mother's pelvis or birth canal. This can be because of a small pelvis and an average sized fetus, a large baby with an average sized pelvis or because of malposition of teh fetal head.

CPD is one of the commonest cause of different complications in labor, including prolonged labor, fetal distress, and delayed second stage .

Cephalo-pelvic disproportion (CPD) is very frequently diagnosed and is a very common indication of cesarian sections, especially when there is failure to progress in labor.

But it is very difficult to diagnose CPD before a women has started her labor pains since it is very difficult to anticipate how well the fetal head and the maternal pelvis will adjust and mould to each other.

Causes of Cephalopelvic Disproportion (CPD):

Increased Fetal Weight:

  • Very large baby due to hereditary reasons - a baby whose weight is estimated to be above 5 Kgs or 10 pounds .
  • Postmature baby - when the pregnancy goes above 42 weeks.
  • Babies of women with diabetes usually tend to be big.
  • Babies of mothers who have had a number of children - each succeeding baby tends to be larger and heavier.

Abnormal Fetal Position:

  • Occipito-posterior position - In this position the fetus faces the mothers abdomen instead of her back.
  • Brow presentation
  • Face presentation.

Problems with the Pelvis:

  • Small pelvis.
  • Abnormal shape of the pelvis due to diseases like rickets, osteomalacia or tuberculosis.
  • Abnormal shape due to previous accidents.
  • Tumors of the bones.
  • Childhood poliomyelitis affecting the shape of the hips.
  • Congenital dislocation of the hips.
  • Congenital deformity of the sacrum or coccyx.

Problems with the Genital tract:

  • Tumors like fibroids obstructing the birth passage.
  • Congenital rigidity of the cervix.
  • Scarring of the cervix due to previous operations like conisation.
  • Congenital vaginal septum.

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Diagnosis of Cephalopelvic Disproportion (CPD)

Diagnosis of CPD is very difficult. This is because it is difficult to estimate exactly how much the mother's ligaments and joints will 'give' or relax before labor starts.

The fetal head also has a great capacity to mould - the skull bones can overlap to some extent and decrease the diameter of the head. So, a baby who appears to be too big to pass through its mother's birth passage before labor, may do so without much problem when active uterine contractions start.

A 'trial of labour' should always be given to all women with average sized pelvis and an average sized fetus even if the pelvis appears apparently too small for the baby.

An estimation of the size of the pelvis can be made by two methods:

  • Clinical Pelvimetry:

    The assessment of the size of the pelvis is made manually by examining the pelvis and palpating the pelvic bones by vaginal examination. It is usually carried out after 37 weeks of pregnancy or at the time of the onset of labor.

    The entire bony arch of the mother's pelvis, including the sacrum , the sacro-coccygeal joint, the sacro-sciatic notch, the ischial spines, the ilio-pectineal lines and the pubic arch are palpated and an assessment of the size of the pelvis made. The diameter of the pelvis is measured with the index and middle fingers of the hand.

  • Radiological Pelvimetry:

    Xrays or CT scans are taken of the pelvis in different angles and views and the pelvic diameter measured. But this method is not done nowadays as it can cause radiation toxicity to the baby.

  • Ultrasound:

    The estimation of the baby's size can be made by ultrasonogram and an assessment of potential CPD can be made when the results are compared with the clinical pelvimetry .

Treatment of Cephalopelvic Disproportion (CPD):

If the surgeon is absolutely certain that there is cephalopelvic disproportion, then a Cesarian section is the only option to deliver the baby.

However, women who have an average sized baby and and an average sized pelvis or even women in whom vaginal delivery is doubtful, should always be offered a 'trial of labor'.

If, after sufficient time has passed in labor without a delivery, and symptoms of prolonged labor or fetal distress begins to develop, a cesarian section needs to be carried out.

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