Why don't the children have birth mothers?

In Lois Lowry's dystopian novel The Giver, the characters do not have birth mothers in the normal sense. Rather than staying with and being raised by their biological mothers, children are assigned to a family unit. In fact, the role of "birthmother" is a job that is assigned to female individuals when vocations are handed down at the Ceremony of Twelve. These women are known as "Vessels," and they are not permitted to have children of their own or spouses after their procreative role is complete. They serve in their pregnancy capacity for three years and then transition into work as Laborers. This vocation is considered low status. 
Another interesting fact is that birthmothers are not allowed to see or interact with the children they give birth to in any way. This is ultimately done to further repress the feelings, emotions, and human connections (which may result in pain and unpleasantness) within the Community.