Prenups and postnups: how they hold up in divorce

The prenup conversation is the one nobody wants to have. It happens — when it happens — in the months before a wedding, often awkwardly, often after one set of parents has pushed for it. The postnup version is even harder: it happens during a marriage, sometimes after a crisis. Both documents serve the same function: pre-committing to what happens financially if the marriage ends. Both are increasingly common, especially in second marriages, in marriages where one party brings significant assets, and in marriages with substantial income disparity.

What follows is how these agreements actually work in a divorce, what they can and can’t do, and when they hold up.

What they are, and what’s different

Both are written contracts. Both must be in writing and signed by both parties. Both can be challenged at the time of divorce. Both can be the most important document in the case if upheld.

What they typically cover

The common provisions:

• Definition of separate vs. marital property. What you each brought in, what stays yours, what becomes joint.
• What happens to specific high-value assets — a business, a family inheritance, a retirement account.
• Treatment of premarital debt — student loans, business debt, pre-marriage credit cards.
• Spousal support / alimony — sometimes a waiver, sometimes a formula, sometimes a cap.
• Treatment of appreciation — if a separate asset gains value during the marriage, is the appreciation shared?

What they can’t cover:

• Child custody. A court won’t enforce a custody provision. Custody is decided based on the child’s best interest at the time of the divorce.
• Child support. Same reasoning — the court won’t bind a future child’s right to support.
• Anything illegal or against public policy. Provisions designed to penalize one party for filing for divorce, for example, are often struck down.

When they’re enforced

The general rule: if the agreement was made fairly, with full disclosure, with both parties having time to consider it, and isn’t unconscionable at enforcement, courts will enforce it.

The specific factors:

• Full disclosure. Both parties disclosed all assets, debts, and income before signing.
• Independent counsel. Each party had their own attorney (or at least had the chance to). Single-attorney prenups draw more scrutiny.
• No duress. The agreement wasn’t signed under pressure — usually meaning weeks or months before the wedding, not the day before.
• Substantively fair at signing. The terms weren’t outrageous when agreed to.
• Not unconscionable at enforcement. The terms aren’t so one-sided in practice that enforcing them would produce a result no reasonable person would have agreed to.

When they don’t hold up

A short catalog of what gets prenups thrown out:

• One party hid assets at signing. Full disclosure isn’t optional.
• The agreement was signed under duress — most commonly, the day before a wedding with travel plans, gifts, and family already committed.
• One party didn’t have meaningful access to counsel — sometimes literally not given time, sometimes given a lawyer hand-picked by the other side.
• The terms have become unconscionable — the most case-by-case of the standards.
• The agreement violates state public policy — varies by state, but provisions waiving child support, penalizing fault, or restricting religious practice are common targets.

State law varies substantially. Some states (like California) hold prenups to a higher standard; others apply ordinary contract principles. Whichever state’s law governs is usually set in the agreement itself.

When a postnup is the right tool

Postnups have specific use cases:

• One spouse comes into a significant asset during the marriage — a business, an inheritance, a settlement.
• A reconciliation after a near-breakup, where the parties want to define what happens if the second crisis breaks the marriage.
• A career change that shifts the financial picture — one spouse leaving work to raise kids, or returning after years away.
• Acquisition of high-risk assets — a business stake, a startup, a high-debt purchase.

Postnups draw more enforcement scrutiny than prenups because the parties are already married, which means they owe each other fiduciary duties. A one-sided postnup looks more like coercion. The same fairness factors apply, with extra weight on disclosure and independent counsel.

How an agreement shapes the actual divorce

If the prenup is valid and applicable, the divorce is substantially simpler. The court doesn’t redo what the parties agreed to. Property gets divided per the agreement. Support is whatever the agreement says (or its absence, if waived).

If the prenup is being challenged, the divorce gets more complicated. There’s usually a separate proceeding — or a major phase of the divorce — addressing whether the prenup holds up. Months added. Tens of thousands of dollars in attorney time. And the outcome is binary: if upheld, it controls; if not, the case is litigated as if no agreement existed.

What to do if you have one

A short checklist when a divorce starts:

• Get a copy of the agreement.
• Identify which state’s law governs.
• Check the signing date — duress claims often turn on how close to the wedding it was signed.
• Look for the financial-disclosure schedules attached. Missing schedules are a common ground for challenge.
• Bring it to an attorney for an enforceability assessment before negotiating anything.

A valid prenup is the cleanest fact pattern in a divorce. A challenged one is among the messiest. Knowing which you have shapes the entire case.