I am pregnant and wish to give custody to the father before the child is born

You have to wait until the child is born so that a paternity test can be performed, which will show that he is indeed the father. Then contact an attorney; tell him or her what exactly you want(to be give up custody, placement, child support, etc.). They can draft a document for you.

Nothing can be done until the child is born.

A paternity test won't have to be done if the father acknowledges paternity and signs the birth certificate.
You both then need to petition in court for the custody arrangement at which time he can get full custody with you having visitation rights and pay child support.

You could get with an attorney now and start getting the paper work together, but nothing can be filed until the child is actually born.