It's also important to figure out what kind of relationship your partner has with their ex, because that could cause a little stress down the line. "If they're still in the process of working out custody, or if each parent is constantly attempting to undermine the other, that's a situation that can cause a lot of grief in a growing relationship," Dr. Michael says. "It's a big red flag." That said, not every co-parenting situation is going to be a negative one. Your partner might be extremely close with their ex, which can inadvertently lead to a little jealousy down the line — whether that jealousy is coming from you or the ex. "You've got to be able to keep your feelings in check," Dr. Michael says. And if you're struggling, usually the best thing to do is to talk to your partner about your feelings.