A premarital agreement, also called antenuptial agreement, is a written contract entered into in contemplation of marriage. A prenup can protect your separate property and address the division of assets in the event of divorce or death of a spouse. Findlaw has a short article listing the top 10 reasons a premarital agreement may be found invalid. The original article has more details on each point that is worth reading through, but here are the reasons in short that a prenup may be invalidated:

  1. NO WRITTEN AGREEMENT
  2. NOT PROPERLY EXECUTED. 
  3. YOU WERE PRESSURED. 
  4. YOU DIDN’T READ IT.
  5. NO TIME FOR CONSIDERATION. 
  6. INVALID PROVISIONS. 
  7. FALSE INFORMATION. 
  8. INCOMPLETE INFORMATION. 
  9. NO INDEPENDENT COUNSEL.
  10. UNCONSCIONABILITY.  

Each state has very specific laws about prenuptial agreements. In New Hampshire, a prenup is generally presumed to be valid unless a party proves circumstances such as 1) fraud, undue influence, duress; 2) failure to disclose a material fact; 3) that the agreement is unconscionable; 4) or circumstances have rendered it unfair. In order to protect the validity of any prenup, parties desiring a prenuptial agreement should contact a seasoned New Hampshire attorney.

Source: Attorney Stephen Worrall’s Georgia Family Law Blog post "Top Ten Reasons a Premarital Agreement May be Invalid"