The pelvis is a bony basin through which body weight is transmitted to the lower extremities. A woman’s pelvis is adapted for childbearing. It is made up of four bones that join to form a passage that will allow your baby to be born.
The pelvis is divided into the true and false sections. The false pelvis (although essential to support the abdominal contents) is of no importance in pregnancy. The true pelvis consists of the bony birth passage. For a vaginal delivery, this passage must be of adequate size and shape to allow the baby to pass through.
During pregnancy (under the influence of the hormone relaxin), there is a certain amount of softening and stretching that occurs in the connective tissue of the pelvic joints.
This results in slight separation between the bones in the front, allowing a little “give” in the pelvis – which then allows the baby’s head to move through. The joints at the back (which are known to cause pain) are the sacro-iliac joints.
There’s a good deal of strain in these joints towards the latter part of pregnancy that usually manifests as low backache.
In normal labour the baby’s head is the first part to enter the pelvic passage – specifically the bony ring of the inlet (or entrance) to the pelvis. It then moves through the part of the pelvis known as the cavity (or mid-pelvis) and finally emerges through the outlet (or exit).
Each pelvis is classified according to the shape of the inlet. The gynecoid pelvis is ideal for birth. The platypelloid shape allows the baby to enter only in a lateral position.
The android pelvis (male shape) may force the baby to engage in a posterior position. The anthropoid shape also tends to result in a posterior position. These positions usually result in a longer labour and a delivery that may require intervention or even a c-section.
The size and shape of your pelvis is important because of the risk of disproportion – or “bad fit”. However, unless the mother has had a problem with her pelvis due to previous injury or the baby expected is oversized; there is usually no need to be concerned that a vaginal delivery will not be possible.
The pelvis in labour
Only during labour (as the mother adopts the best positions to facilitate the baby’s descent), will the pelvis open up to maximum. How much extra room the pelvis will then have cannot be determined before labour starts.
By remaining upright you will assist gravity to bring the baby down into the pelvis and by squatting, the long femur bones will act as levers and pull the pelvis open – thus widening the pubic arch. The coccyx will move back out of the way, leaving a clear run for the baby. This increases the cavity of the pelvis by a few centimetres. Remain as active as possible during your labour.