Hooter is correct in that state of residence plays a large role in the matter. However, generally speaking, post-nups are typically upheld more often than pre-nups. The logic being that there has been more time and opportunity for factual discovery within the context of marital issues, such as financial holdings, income, etc.
Regardless of state or generalities, one thing that's universal is that fraud trumps all. If there were material non-disclosures prior to contract execution or other fraudulent aspects of the situation, then you may have a shot at overturning it.