 most usual position is described as left or right occipito-anterior, in other words the fetus is coming head first, and has his back to the pubis; this is the least space occupying position. “Face to pubis” is undesirable but generally accepted; “breech,” coming bottom first was accepted until recently, but may to some obstetricians be an indication for a Caesarean section. And then there all the thoroughly undesirable presentations of a leading arm or a leading leg or lying transversely.
Another problem in the canal is with the umbilical cord which may be prolapsed, meaning it is coming ahead of the fetus, resulting in compression of the oxygen supply to the fetus as the cord is compressed by the head against the vaginal wall, or may be wrapped around the fetus’ neck so the further down the canal the fetus goes, the tighter becomes the cord.
Within the canal the process of birth may not progress because of uterine inertia or may not progress because of obstruction among the commonest cause now is Shoulder Dystocia, where the leading shoulder jams under the pubis and blocks progress.
Lack of Oxygen (perinatal asphyxia)
The failure of an adequate oxygen supply may be indicated by the fetal distress in utero, when the placenta is probably inadequate, or may occur due to compression of the umbilical cord during progress down the birth canal. Prolonged labour may result in fetal death.
The APGAR immediately after birth assesses the evidence of brain injury from lack of oxygen, but the long term effect, cerebral palsy, remains to be shown in the course of time.
Passive Trauma to the Head in the Birth Canal
The part of the scalp that leads down the canal (presenting part) is likely to be swollen (caput succedaneum), and this resolves in a few days; a prolonged delivery is usually marked by this, whereas a Caesarean section is free of it. Compression of the head may cause some bleeding beneath the skull periosteum (cephalhematoma) which is rarely of consequence. Fractures may occur without interference by attending staff as a result of compression of the skull against the bony parts of the canal. There may be some internal bleeding, more likely to be found in premature infants, possibly associated with hypoxia (low oxygen). Sudden relief of pressure on the head, as in a precipitate delivery, may result in intracranial bleeding – a quick delivery may seem to be desirable but in fact is dangerous. There are three layers of membranes between the brain and the skull, bleeding may be found between these layers, or within the brain’s cavity (ventricle) or less often, within the substance of the brain.
Some hemorrhage into the eye, sub-conjunctival, or retinal may occur with difficult or precipitate deliveries
Passive Trauma to the Brachial Plexus in the Birth Canal
The brachial plexus is a mesh of interconnecting nerve roots, located at the base of the neck, and liable to be stretched if the arm is pulled down or the head is pulled sideways. This may occur with damage to the plexus during a difficult delivery, as for instance shoulder dystocia, but since it is also found when there has been a Caesarean section, it is not reasonable to assume automatically it was the result of force.
The phrenic nerve to one side of the diaphragm may be paralyzed, but usually recovers spontaneously.
If force is applied with forceps or by vacuum extractor, it is inevitable that some mark will be left. In general these marks subside in a few days, and are a small price to pay for the easing of a difficult labor.
Occasionally there will be more significant injury such as a fracture of the skull, or an injury to the facial nerve; injury to this nerve is not necessarily iatrogenic, and may have occurred due to other unrecognized factors during birth.
There is a potential for spinal vertebral or cord injury by extreme manipulation, traction with a breech delivery or rotation with a difficult vertex delivery; the spinal cord has very limited opportunity for self-repair.
Fractures of long bones, humerus and femur, may occur as a result of forced manipulation