Why the head of the foetus normally comes out first ?

The head is the only inflexible bit of the baby. The rest of the body is flexible to squeeze through the narrow birth channel. The best position is head come out first and facing the mother's tail bone. If the back of the head facing the tail-bone there is still a little obstruction.
When the baby's head emerges whole, the midwive usually will tell the mother to stop pushing and check if the umbilical cord is tied around the neck and may strangle the baby. Usually there will be enough room to untie the baby before progressing to totally delivery. The worse scenario is to pull the baby out with some instruments without permanantly injuring him.
However, if the baby comes out in a position other than head first, and if the umbilical cord is caught anywhere between the stages, it is difficult to pull on the baby without seriously injuring him, since the inflexible head is difficult to come out. The cord may be caught around the neck and is much more difficult to maneuvre.

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