A premarital agreement is something that you and your future spouse need to discuss. The terms have to be mutually protecting, and the document can't favor one person. It is necessary to ensure that you have everything in order with it because any errors can mean that it is thrown out when it is time for you to call on the protections it provides.
Because this document can't be signed in a rush, you should discuss it as early in the engagement as possible. This allows time to negotiate the terms and for both parties to thoroughly review it to ensure that it is set up in the agreed upon manner.
One thing that you need to remember in these cases is that your prenup must be based solely on facts. Both adults have to make full disclosures about their financial health. Leaving anything out can mean that it isn't recognized by the court.
You can't include anything about child custody or child support in the premarital agreement. This is because these matters have to be based on what's best for the child, and you can't know what the future is going to hold.
It is shocking to some people who are getting married that prenuptial agreements can help to protect marital assets. If one spouse is coming into the marriage with debts, the prenuptial agreement can stop creditors from being able to claim marital property but there must be a limitation of debt liability clearly noted in the prenup.
These documents are fully customized, so make sure that it is set up exactly how you want before you sign anything. If you discuss this early enough, there will be time to get it all worked out.