I heard that a prenuptial agreement signed within 48 hours before a wedding is not valid. We were married two hours after we signed ours.
You can relax and enjoy your newly wedded bliss. Probably.
Generally, prenuptial agreements are legal if both parties are fully informed about the underlying facts of the agreement and what it means.
In addition, if one prospective spouse is less sophisticated than the other when it comes to financial issues, he or she should receive the advice of a lawyer. If an agreement is signed under duress, and the person coerced into signing is unsophisticated and has no legal help, a court is likely to disregard it. Duress in this case means a pre-aisle threat such as: I won't go through with the ceremony if you don't sign here.
So, while signing at the last minute may cause a court to take a closer look at your prenup if one of you wants to enforce it later, it won't by itself make the document invalid.