Only 1 to 2 percent of married couples request postnuptial agreements, compared with 5 to 8 percent of engaged couples requesting prenups. But lawyers and estate planners report increased interest over the past five years.

Postnups are used to protect a sudden windfall, or to care for children from a previous marriage. Some married women use them after stepping out of the workforce to protect their financial future.

They're tough to pull off. Both spouses must buy in, and each must have legal representation. The bottom line: Is this midmarriage reconfiguration fair and honest? They cannot be used to alter child support.