Can we throw out a prenup once we have children?

My fiancée and I are to be married in 3 months and I think I need a prenup. I have a lot more assets than she does and I earn a lot more. I’ve made it clear to her that one of the primary reasons I’m willing to get married is because I want a son to carry on my family name. If we have a son, I’m willing to forget the prenup and share whatever I have with her if the marriage doesn’t work out, because I would want her, as my son’s mother, to be taken care of. Can we include something in the prenup that states that it is only valid until she gives birth to a boy and if she does, the prenup goes away at that point? Can I draft this type of agreement myself or must a lawyer do it?