A prenuptial agreement is signed before the wedding and takes effect on the marriage date. A postnuptial agreement is signed after the wedding and takes effect the moment both spouses sign it.
That single difference in timing affects bargaining power, how strictly courts review the contract, and when the agreement becomes binding. Both documents can control property, debts, and (in many cases) spousal support but neither can limit a child’s right to support.
This guide covers both agreements for US couples in general. State rules vary, so the details below are a starting point and not legal advice.
What is a Prenuptial Agreement?
A prenuptial agreement (also called a premarital agreement) is a written contract between two people who plan to marry.
It becomes effective on the wedding day. It decides who owns what and who owes what if the marriage ends by divorce or death.
Under the Uniform Premarital Agreement Act (UPAA), the contract must be in writing and signed by both parties, and it becomes effective on marriage. After the wedding, it can be changed or cancelled only by a written agreement that both spouses sign.
Without a prenup, state law decides how property is divided. A prenup replaces those default rules with the couple’s own terms.
Prenups were once seen as a tool for the wealthy. That has changed. About 1 in 5 married couples has one, and the average age at first marriage in 2025 was 28.4 for women and 30.8 for men, more than three years older than in 2005. Couples who marry later usually arrive with savings, retirement accounts, student loans, or a business.
What is a Postnuptial Agreement?
A postnuptial agreement (also called a marital agreement) is a written contract between spouses, signed during the marriage and before any separation or divorce case. It does the same job as a prenup: it sets property rights, debt responsibility, and often spousal support.
The timing rule is different. A marital agreement takes effect when both spouses sign it, while a premarital agreement takes effect on marriage.
Couples sign postnups for several practical reasons:
- They never signed a prenup and now want written rules for their money.
- One spouse received an inheritance or started a business and wants it kept separate.
- One spouse is leaving paid work to raise children, and both want a fair plan for that choice.
- They already have a prenup and need to update it, since a valid postnup is the way to amend a prenup after the wedding.
Prenup vs Postnup: Key Differences
A prenup and a postnup cover the same subjects, but they differ on timing, leverage, and court review.
The table below shows the differences in one place.
| Feature | Prenuptial Agreement | Postnuptial Agreement |
|---|---|---|
| When it is signed | Before the wedding | After the wedding, during the marriage |
| When it takes effect | On the wedding day | When both spouses sign |
| Bargaining power | Either person can still walk away from the wedding | Spouses are already married, so leverage is harder to measure |
| Court review | Contract rules plus state premarital law | Contract rules, and stricter review in many states |
| Typical reason | Protect assets and debts brought into the marriage | Fix a gap, respond to a life change, or update a prenup |
| Child support | Cannot reduce a child’s right to support | Cannot reduce a child’s right to support |
| Lawyers | Each person should have their own | Each spouse should have their own; some states require it |
How Courts Review Each Agreement Differently
Courts apply contract rules to both agreements, but many states examine postnups more closely.
The reason is bargaining power. Before a wedding, either person can refuse to sign and leave. After the wedding, spouses owe each other duties and a refusal can strain the marriage.
The Uniform Law Commission noted in 2013 that many states apply different standards to premarital and marital agreements, with higher burdens on the spouse who wants to enforce a marital agreement.
Its Uniform Premarital and Marital Agreements Act (UPMAA) treats both under one set of rules. Colorado and North Dakota enacted it in 2013.
Several states show how far the standards can differ:
- Minnesota: A postnup is valid only if each spouse had separate legal counsel at signing. It is presumed unenforceable if either spouse starts a divorce or separation case within two years, unless the spouse enforcing it proves it is fair and equitable.
- Connecticut: The state supreme court upheld postnups as consistent with public policy and applied standards stricter than those for prenups.
- Ohio: A postnup is unenforceable unless both spouses understood the value, nature, and extent of all property when they signed. It also fails if it encourages divorce.
Some states may also reject a postnup signed shortly before a divorce filing. A spouse who signs a postnup late in a failing marriage should expect a challenge.
What a Prenup or Postnup Can and Cannot Cover
Both agreements can control property, debts, and often spousal support. They cannot control a child’s right to support.
The UPAA states that a child’s right to support may not be adversely affected by a premarital agreement.
Child support terms in a postnup that deal with future obligations are generally unenforceable at a later divorce. Custody is decided by a court based on the child’s best interests, so a contract cannot fix it in advance.
Two more limits apply under the UPMAA. Courts may step in when enforcing an agreement would cause undue hardship, and terms that penalize a spouse for filing for separation or divorce are unenforceable. Waivers of spousal support get the most scrutiny.
When a Prenup Makes Sense?
A prenup makes sense when at least one partner brings assets, debts, or obligations that the couple wants treated in a specific way.
Signing before the wedding gives the strongest legal footing, because the agreement is made when both people are free to walk away.
Situations that commonly call for a prenup:
- One partner owns a business or a share in one.
- One or both partners have children from an earlier relationship and want to protect their inheritance.
- One partner has large student loans or other debt the other should not share.
- One partner expects a family inheritance or owns real estate before the marriage.
- The partners marry later in life and have already built up savings and retirement accounts.
When a Postnup Makes Sense?
A postnup makes sense when a couple is already married and their finances have changed or were never put in writing.
It is the only route to a marital contract after the wedding, and the only way to amend an existing prenup.
Common reasons include a new business, a large inheritance, a move to another state, or a decision that one spouse will stay home with children.
A postnup also helps when one spouse wanted a prenup but the couple ran out of time before the wedding.
Because courts review postnups closely, the process matters more. Each spouse should have a separate lawyer, and neither should sign under pressure or in the middle of a dispute.
How to Make a Prenup or Postnup Hold Up in Court
An agreement holds up when it is fair, fully disclosed, and signed freely. Courts strike agreements that were rushed, one sided, or built on hidden information. Follow these five steps to reduce the risk.
- Exchange complete financial disclosure. Each person lists all assets, debts, and income in writing. Inadequate disclosure is a standard reason courts refuse to enforce an agreement.
- Hire separate lawyers. One lawyer cannot fairly represent two people with opposing interests. Minnesota makes this mandatory for postnups.
- Start early. Begin a prenup months before the wedding. A last minute agreement invites a claim of duress.
- Put everything in a signed writing. Follow your state’s formalities, such as notarization or acknowledgment, and keep a copy for each spouse.
- Keep the terms fair and review them. An agreement that leaves one spouse with nothing is easier to challenge. Revisit it after major events such as a birth, a sale of a business, or a move.
Conclusion
A prenup and a postnup do the same job at different times. If you are engaged, a prenup gives you the cleanest legal footing. If you are already married, a postnup lets you write down rules for your money, and it can update an older prenup.
The stricter court review of postnups is the main reason to take the process seriously. Disclose everything, use separate lawyers, and give yourselves time.
Rules differ by state and change over time, so speak with a licensed family law attorney in your state before you sign anything.
