There are several reasons a woman would miscarry a child - some are her own fault, others not. A baby is born to a specific mother for a specific reason. If during the pregnancy the mother were to fall and injure herself in such a way that the baby inside her was damaged to a point where the physical body could not be effectively used any more as a vehicle for the spirit for that intended life, the spirit would withdraw and wait for a better opportunity. This does not only hold true for an injury to the mother. If in any instance the baby is harmed beyond its needs, it has no choice but to withdraw. In almost every instance the mother falls pregnant again a few months down the line and gives birth to a beautiful baby. For the rest of her life she mourns the loss of the first baby and celebrates the life of the second, not realising it is the same baby simply returning when circumstances had improved.
Another instance is if the change in lifestyle of the parents is not ideal anymore for the lesson to be learnt. If the lesson to be learnt was humility, you could not be born to a king, for example. But, if during the pregnancy the mother met a king and he proposed marriage to her and she accepted the baby would be born to a king, that wouldn't do for the lesson and the life would be wasted. In that case it would be better for the spirit to withdraw and find a more suitable family for the lesson.
Sometimes the spirit becomes fearful of the coming life with all it holds and changes its mind - it is entirely within its right to do so. In the spirit world nothing is ever denied to anyone, if you wish to return to earth in a life, you may - if you choose to change your mind you may, nothing is ever refused to any soul.