Hello - big hugs to you! It sounds tough, and difficult to know how best to deal with things.
I would like to add the disclaimer immediately that have no idea what I am about to suggest is the 'right' way to deal with it, but when my DS (2) has occasionally done similar things to me I have reacted (completely instinctively!) with self defense moves first and foremost to physically prevent him from hitting me (before explanations about not hitting etc. once the immediate threat has stopped and he has calmed down). Maybe this could be helpful for you both in terms of your pregnancy (far too important to risk!) and also showing that you will not tolerate being hit. After all, why should any of us, even (or perhaps especially) from our own child?
Just tonight, when DS was a bit over tired and about to go to bed, he came at me pretty quickly from the side on and tried to hit me with what would probably have been quite a bit of force on my head (I was sitting on the carpet with DD). I should add that this is not really commonplace but does happen sometimes when he's tired or frustrated. I didn't even think about it (I could only half see him becuase of the angle he was at and barely had time to react) but instinctively moved my arm around in a 'blocking' move which resulted in him not making contact with me at all but his arm being pushed away and him being moved (not roughly!) away from me. He was left sitting on the sofa looking a bit confused and not very happy that his assault had been foiled!
Please don't for a minute thing I am advocating any kind of violence or 'hitting back' type response - I am honestly the most hippy / pacifist / pro-gentle parenting types around! BUT at the same time I am absolutely buggered if I am going to just let anyone hit me without stopping them from doing it (I may be a hippy but am a bit of a feisty redhead one!) :-D
DD saw what happened tonight and seemed quite impressed (she has been on the recieving end sometimes when her brother is at it!) In the past I have tried to teach her some very basic self defense too (although I always emphaise that hitting is wrong / not to hit back / only do enough to stop someone from hurting you etc. - obviously don't want to turn it into a physical fight or put herself in any danger!) I just really want her to know how to protect herself if cornered or attacked unaware by a (strong) 2YO terror, as she has been before now. Anyway, I digress...!
I am sure someone will tell me if this is in any way really bad advice, but I do wonder whether (as you mentioned gentle parenting etc - which don't get me wrong, is usually my own approach too!) that you may have just been just a bit passive about actually allowing the hitting to happen rather than physically stopping him in his tracks? It takes a speedy reaction time to get it right but there seems (to me anyway!) to be a huge difference between taking a gentle approach and actually putting up with violence against you... If you were able to hone your reflexes to block / stop him before he could hurt you, then do the gentle explanation thing later (once he has calmed down) could that possibly work? It might make him realise that you are simply not going to tolerate being hit / kicked / hurt and that him trying will have no effect because you will simply prevent him from doing it and walk away...
This is only a suggestion of course as I have found it to be useful in my own circumstances... The very best of luck with it all!