In my experience, when one has been let down in relationships, be it boyfriends or friends etc, trust is a BIG issue...when you find "the one" you'll push and push them to see if they remain loyal and stay with you through anything...then the moment they've had enough and walk the other way, one will always state "I knew it, just like all the rest of them".
How I know this? Cos it was me at one stage. It started off with being badly let down by my oh so lovely stepfather, then mother (but I don't really blame her tbh, just disappointed), then fell into crap after crap relationships with blokes who were no good. Have been seriously let down by my siblings and so called friends...then I find my OH and I have pushed him so far so many times in the past that its a wonder he's still with me! I even left him for a year cos I couldn't handle me! Now though, I don't push him near as much, but don't get me wrong, I won't take crap of him, cos he ain't perfect either, but I realised that 90% of the time it was me getting in the way of our relationship.
Hun, it doesn't sound like you guys need to split up. Its just trust issues that need to be worked upon. And these can be exaggerated cos of pregnancy hormones etc. Throughout my current pregnancy I've felt like squealing like a banshee at my OH and have blown lots of the daftest stuff out of proportion when usually I'd let it go over my head.
If he didn't want to be with you, he wouldn't be talking about you to other gals. And the flirtacious friend...god she sounds like one of my sisters...why would your OH want someone like that over you who he thinks is beautiful and is carrying his LO! I get what your saying about the FB pic he's been tagged in and its causing you to think all sorts, but don't you think if he had something to hide then he would ensure that there'd be no mission you could see his FB profile etc?
Aww hun, I do totally understand where you're coming from though, but I think you just need to get him to give ya a big

and reassure ya that alls ok. Hope you can work through it. x