The female pelvis consists of seven segments connected by tendons and other tissues. It is not a rigid bone ring. In addition, the hormone Relaxin during natural birth ensures that the stabilizing tissue connections softens. This creates space for the baby.
To make us of this softening, an upright birth position is important. The diagnosis head-pelvis-disproportion (Cephalopelvic Disproportion – CPD) does not play a big role in cultures in which people work and are born in squatting position. In Germany, however, most births are still taking place in supine position (birthing women lying on the back). This is convenient for the obstetricians, though more difficult for mother and baby.
How upright postures make birth easier
If the baby pushes through the pelvis, the mother’s tailbone and sacrum move backwards and give the child extra space. However, if the woman is lying on her back or in a semi-sitting position, her body weight is on the sacrum and tailbone, which inhibits a movement of the bones out of the way.
The weight of the child
The argument that a child is too heavy and too big to fit through the pelvis seems questionable. Why should a heavier child have a bigger head than a lightweight child? In addition, nature has provided that the child's head can be compressed during birth. The skull plates are not yet grown together and can slide over each other to facilitate the heads passage through the pelvis. After that, they go back to their original position.
What can influence this kind of diagnosis?
If surveys during pregnancy do not take into account the considerable mobility of the pelvis and the child's head, it is diagnosed that the pelvis is too narrow and therefore a vaginal birth impossible.
If the diagnosis is made during childbirth, most likely the natural hormonal processes have been disturbed or disabled.