Dear R.,
Un-friend them on facebook.
I'm not trying to make light of your feelings, but you are pregnant and you might be feeling more emotional about this than you normally would.
It sounds like your ex betrayed you by leaving and this woman betrayed you by wanting to be "friends".
That's their drama now.
It doesn't have to be yours.
You know, the thing about first loves is that they often don't end up being your TRUE love. Statistically, it just doesn't usually work out as a forever thing, sad to say.
You have a new family now. You are happy. Your husband has given you things your first love wasn't capable of. You have gotten the better end of the deal!
Never doubt that.
To cheer you up, I will tell you about a friend request I got from a guy I went to high school with. He was a senior when I was a freshman. My parents knew him, he was the cousin of one of my best friends so he was very much part of my life. I hadn't heard from him in ages so I accepted him as a friend and didn't think anything of it until the next day when I wanted to send a personal message.
I almost had a heart attack when I saw his pictures that he posted.
He's 51 years old, spread eagle in a g-string holding a guitar because apparently he thinks he's a sex god rock star of some sort.
I am not a facebook expert, but I called a friend to ask how to get him deleted before any of my family members saw any of that and had a coronary.
I could not get him deleted fast enough!
Un-friend them. Go on with your happy life and try not to dwell.
Don't give them the satisfaction of knowing it even bothered you.
That's just my opinion.
Take care of yourself and your growing family.
Best wishes.