This is the term applied to the change in shape of the fetal head which takes place as it passes through the birth canal. It is brought about by pressure between the fetal skull and the maternal pelvis. It results in compression of the movable bones and elongation of those which are not compressed. Moulding brings about a considerable reduction in the size of the presenting diameters while the diameter at right angle to them elongates. This is possible because of the sutures and fontanelles on the vault which allows slight degree of movement and the bones to override each other. In normal vertex presentation, the biperiatal diameter, sub occipito bregmatic reduce while the mentovertical lengthens. During moulding the anterior parietal bone override the posterior one, the frontal and occipital bones go under the parietal bones. The advantage of moulding is that it is a protective mechanism and prevents compression of the fetal brain, once it is not excessive, too rapid or unfavourable direction. The skull of a preterm baby may mould excessively while that of post mature does not mould which tend to make labour more difficult.
In certain types of moulding the internal structure maybe damage given rise to oedema or haemorrhage and congestion may give rise to mild cerebral irritation.
This can lead to death or permanent brain damage. These dangerous moulding includes:
1. Excessive moulding: In cases of prolonged labour, due to cephalo pelvic disproportion, prematurity.
2. Upward moulding: Occipito posterior position resulting in “face to Pubis” and after coming head of the breech .
3. Rapid moulding: Precipitate labour Rapid compression and decompression result in rupture of cerebral membranes. Any baby with any of this dangerous moulding should be cot – nursed and observed for 24hrs for signs of cerebral irritation.