The phrase between parentheses is a critical issue, since it is extremely easy - and, in fact, the default - to cheat without producing illegitimate offspring, thus making the prenup fairly worthless.

It's actually likely to make things worse, since swearing that "I will not cheat and get pregnant" is going to bring one's attention to the backdoor - i.e. that you never promised not to cheat outright. It looks like a classical and extremely clumsy deception.

Unless, that is, you'd be happy with your husband being suspicious of you at the same time he is confident that his child is really his.