If you're considering leaving your job to become a stay-at-home mom, please do yourself—and your marriage—a favor: Get a postnup. You and your husband should sign a postnuptial agreement, one that clearly specifies how you would be compensated for forfeiting your highest-earning years should the marriage end in divorce.

In happy times, staying at home with the kids may seem reasonable, comfortable and manageable.

But what happens if the marriage hits the skids? Will a divorce settlement include compensation for the wife who left her career to be home with the kids? And when she's a single woman again, will the stay-at-home mom be able to slide right back into a job like her old one (with similar pay and benefits)?

Don't count on it.

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Everyone has heard of prenuptial agreements. A prenuptial agreement (or "prenup," for short) is a legal document signed before a couple is married. It details what the couple's property rights and expectations would be upon divorce.

A postnuptial agreement can accomplish those same goals, but as the name implies, a postnup is negotiated and signed after a couple is already married.

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Why is it so important for a stay-at-home mom to have a postnup? Here are my top three reasons:

Prenups and postnups help form the foundation of a happy marriage.

I understand how, at first, it may be difficult to believe that negotiating a postnup could actually make your marriage happier, but it's true.

"Money" is often cited as one of the top reasons for tension in a marriage. It's extremely difficult to agree about what to spend, how much to save, when/where to invest, etc. Why add arguments about career paths and earning potential to this volatile mix? Have those difficult conversations now, while you have a clear head—not years down the road, when you're feeling frustrated after reading about a colleague's big promotion while you stayed up all night with a kid who has a stomach bug.