Q I need advice. When I met my ex-boyfriend, he said he was divorced, but I found out that he was only separated. He said he was going back for the kids; he has two. Now, there’s an even bigger problem. I found out I am pregnant with a little girl. I’m 6 months along. So, basically, I am going to have to co-parent with someone who is married to someone else. What’s good ex-etiquette?

A I understand why you are feeling so alone, but I must call your attention to the fact that most people who co-parent after a breakup co-parent with someone who is married or living with someone else.

Your circumstances are a little more sordid in that he lied and then decided to stay with his wife, but the truth remains that after a breakup, most people re-couple. And that means that if you have joint custody, the child you are carrying will be living with her father and his new partner possibly half the time.

It may not be right away. Infants usually stay with their mother more at the beginning just because of things like breastfeeding, but your child’s time with her father will eventually increase. That means your child’s time with her father’s wife will also increase — if they stay together.

There’s a very real possibility that their marriage may not be able to withstand this level of betrayal. But, let’s say they do stay together. The truth remains that he’s still your child’s father.

A child deserves a relationship with both parents. Since dad also lives with someone else, let’s hope his wife will be gracious enough to understand that your daughter had nothing to do with this and do her best not to resent the child.

What all this looks like in real life is three people doing their best to reconstruct a completely different relationship than any of them previously imagined. That may include more interaction than is comfortable right now, but that will be your life once this child is born.

The best way to cope at this point is to concentrate on giving your daughter the best life you can. That’s putting your child first and that’s good ex-etiquette.

Dr. Jann Blackstone is a child custody mediator and the author of “The Bonus Family Handbook.”