In order for a California prenuptial agreement to be enforceable in divorce, it needs to comply with the California Premarital Agreement Act. The Act requires three things: a prenuptial agreement must be entered into voluntarily; it cannot be unconscionable or fail to meet the disclosure requirements of the Act; and it must not be against public policy.
"Voluntary" has a specific legal meaning: the person against whom enforcement of the agreement is sought must have had independent legal counsel before signing, or must have waived that right. In order to waive the right, the person must have received complete written information about the terms and effect of the agreement, and signed a separate document acknowledging receipt of that information and waiving the right to an attorney. There must be a minimum of seven days between the date the agreement is presented to the person being asked to sign it and the day it is actually signed. Other circumstances may also be considered when determining whether a prenuptial agreement was signed voluntarily, such as whether the party who presented the agreement threatened to call off the wedding if it was not signed.
"Unconscionable" means that enforcing the agreement would be very unfair; just how unfair may require a court to decide. "Against public policy" means something that violates the interests of society, such as enforcing a prenuptial agreement that would cause one spouse to become homeless or an agreement that would leave the couple's child without support. The rights of a couple's children or future children cannot be waived in a prenuptial agreement or otherwise.
The best way to make sure a California prenuptial agreement will be enforceable is to have it prepared by a qualified California family law attorney well in advance of the proposed wedding, and to be sure the person being asked to sign it chooses his or her own attorney to review it.