Prenuptial agreements are useful for many things, but figuring out custody arrangements before you have children or before the possibility of a divorce is not something they are meant to address. If you think about it, planning that far in advance would be detrimental to both the children and their parents. The situation the parents face now may be completely different at the time of a divorce. Children grow and change as well, requiring special care and having different needs.
If you are thinking about marriage, you should also be thinking about a prenuptial agreement. These marital agreements are becoming increasingly popular, especially for people who have already been down Divorce Road, those with blended families and others who have high-value assets, such as a business, to protect.