Souls are made up of mana, which is the basic element of life. It allows our bodies and minds to function, and is also the basis for magic in an individual. A person's magical potential is determined by the level of mana they possess when they are born, and is used to perform rituals and spells.
A witch's mana is used to create the soul of a child when it is gestating. Over the ninth month period, more mana is needed to sustain the child as it grows. This saps the mother of her own mana, leading to all of the woes of pregnancy. The mana drain causes her spells to gets weaker until the child is born.
There are a number of problems with this setup works. Because the child's soul is made from his mother's mana, it can logically be assumed that she doesn't get that amount back. This means that witches get weaker the more kids they have. Since witches are at the top echelons of society, losing magic in this way is counterproductive.
How could this setup be made to prevent this from happening?