.... pressed return too early.
You wouldn't be a nice person if you weren't concerned, but on the other hand it's awkward to intervene with someone you don't know well.
If your mutual friend understands the risks and truly cares for this woman, she'd have the difficult conversation. Talk to mutual friend and explain the risks, graphically if necessary.
Otherwise maybe consider sending a private FB message to the pregnant woman. "Sorry, I know this is rude, but when I was pregnant I found out a lot about the risks of drinking to the baby. I might be getting completely the wrong impression, but it seems like you're drinking. Even at the risk of offending you, or getting it completely wrong because you're not drinking, as one mum to another, I just wanted to be sure that someone had told you about the risks. It was difficult deciding whether to write this message when in many ways it's none of my business, but if someone knew that something I seemed to be doing was bad for my child, and I didn't know, I'd want them to tell me."
And yes, I know that she must have been told, but it allows her to save face and gives you an excuse if you assume that she hasn't.
Good luck. You seem like a caring person.