An Overview of the Anatomy and Physiology of the Foetus and the Maternal
Pelvis and Uterus
The unborn baby is usually viewed as a static recipient of uterine forces (contractions) which eventually drive him out of his comfortable home.
This is not true. Imprinted in the genes of a baby is the knowledge of how to enter the world. Research is proving that it is the baby who is the instigator of the birth process. He causes changes in hormonal levels, muscle sensitivity, smooth muscle tension and maternal feelings (e.g. the ‘nesting’ instinct) experienced by some women prior to labour commencing. The baby also plays a definite physical part in getting himself ready for birth. His movements, while trying to enter the pelvic brim during late pregnancy, trigger off Braxton Hicks contractions (sometimes called practise contractions or ‘entering’ pains.)
The human foetus is an awkward shape. His head, in relationship to his body, is relatively large and oval. The longest measurement is from front to back and the widest part of his head is towards the back, known as the parietal diameter. He has a flexible neck which allows him to tuck his head onto his chest if he is facing the correct way. His shoulders are wider from side to side. From these facts it is obvious that the baby has to undergo a lot of turning and bending if he is to successfully pass through the pelvis and into the world.
The angle between the maternal spine and the pelvic brim determines the amount of space in which the baby is able to manoeuvre. He needs as much room as possible. By 34-40 weeks he is no longer able to flip about in his capsule but must move deliberately. However, he is still buoyant and can manage to turn over. When the pregnancy reaches 36/40 weeks, his back is becoming quite long and if he is to get into the optimal position for labour, he must do so now. Second or subsequent babies engage later because their mothers have a rounder uterus and softer abdominal muscles. Sometimes they don’t engage until labour begins.
During an OA labour the foetal head flexes even further so that the smallest part is presenting first (the sub-occipito bregmatic.) This is aided by the folding of the soft cranial bones at the fontanelles, to make the passage through the maternal pelvis easier,
If the foetus is in the OP position, his head is unable to flex – so to pass through the pelvis he has to reduce his head size by way of moulding. This is where the membranous tissue, skin and soft bones are pushed forwards, slowly reducing the diameter of the head until he can squeeze through the pelvic brim. Once in the pelvic cavity, he has to continue moulding to pass the ischial spines before exiting the pelvic outlet. If he can’t get into the pelvic brim during labour, a Caesarean section will be performed. If he can’t pass the ‘spines’, either because he remains OP or because he becomes transverse (when the foetal head turns to the side), a Caesarean section will most likely be carried out. If he is able to squeeze past the “spines” it is highly likely that the foetal head will rotate against the pelvic floor muscles into the OA position because this is the only stage during labour that the foetus has something firm to rotate on. A lift-out forceps (or ventouse) may be required to help birth the baby because of the labouring woman’s weak bearing down efforts (due o maternal exhaustion or from an epidural anaesthetic) or from foetal distress.
Sometimes the foetal head fails to rotate and the baby is born face to pubes. If he can’t be born because he gets ‘caught’ at the pelvic outlet (usually because his shoulders are causing a problem at the brim), a rotation forceps or ventouse is performed to deliver the baby. An episiotomy is usual when forceps or ventouse are used. Occasionally, the baby manages to be born without medical help. This is more common with multigravid women.
The moulding of the foetal head will result in a caput forming over the presenting part. This can be felt during an internal examination. When the baby is born, an elongated forehead will be apparent, accompanied by swelling and bruising. It usually takes about 48 hours or so for the shape of the baby’s head to smooth out and for the bruising and swelling to subside.
© Copyright Jean Sutton