DEAR MR. DAD: My wife and I are about to get a divorce. We have a 1-year-old boy and she’s pregnant with our second. Here’s the problem: She’s been having an affair for the last two years and I’m concerned that the children aren’t actually mine. What can I do to protect myself?

Answer: Hire a lawyer immediately. Then get DNA tests for you and the children. Expect to pay upward of $400 for the testing, but given the horror stories I’ve heard from men in your situation, it’s a worthwhile expense.

Your goal is to avoid becoming a victim of paternity fraud. This is when a mother lies about who a child’s father is for the purposes of monetary gain. In your case, you could be on the hook for 18 or more years of child support for a child or children who aren’t yours.

Paternity fraud is not uncommon, but if you’re early in the process, you may be able to stop paternity fraud before it happens. If the DNA tests show that you’re not the father, ask your lawyer how to challenge paternity. Depending on where you live, you’ll typically have six to 24 months to do so. Don’t let that window of opportunity close.

One man received a bill for $75,000 in back child support. That came as quite a surprise, since he didn’t think he had any children. He tracked down his ex-girlfriend and her child and got a paternity test, which proved conclusively that he was not the father. But because he hadn’t challenged paternity soon enough, he was still held responsible for the arrears.

Unfortunately, even challenging paternity within the window doesn’t guarantee that you’ll avoid being defrauded. In many states, when a child is born during a marriage, the husband is presumed to be the father – biology is irrelevant. So even if DNA shows that he’s not the father, he’ll still owe child support.

Why are the courts making men pay for children who aren’t theirs, whom they may never have met or known about or have no legal rights to see? Unfortunately, it’s big business. Many states receive government funds for every child support claim they issue, and they may take a percentage of money collected – often to the tune of tens of millions of dollars per year.

The big complication here is that you undoubtedly love your son and you may decide that you want to claim him as your own regardless of what a paternity test might show. This means that you’ll need to work out custody and child-support arrangements with your ex. Be aware, though, that if someone else orders a paternity test later (say, she marries the man she’s been having the affair with and wants him to be daddy) and you’re ruled out as the child’s father, you could lose your custodial rights altogether.