Sad situation... but I am going to take the question into a different direction, and I apologize if this does not quite hit the mark for you. Unfortunately, your husband is an alcoholic/addict (I think you already get this, as you wrote about it pretty eloquently). I hate to frame it like this, but the question should be less about what his deal is, and more about what makes you keep allowing him back into your life. The traumas that may be lurking behind the scenes of your history seem to have set the table for a man to come in and be able to treat you this way. If you begin the process of healing yourself, his behavior will get less and less acceptable and you will not place yourself in the same situation again.
Listen, unless he bottoms out, it is pretty clear that he will continue to treat you this way. To protect yourself and to create some natural consequences, you may wish to set a boundary and tell him that treatment is necessary before you will even consider taking him back. And geez, it doesn't seem to matter what your circumstances are - even with you pregnant, he resorted to his same old same old.
That covers the alcoholic part of his presentation... but there seems more to the story. Is it possible that he is also harboring another relationship during the times you two are apart? It would be difficult for me to believe that a male on his own in the city would go for 6 to 10 weeks at a time without entertaining some form of relationship or related activity. Too, the time you are apart (I am just doing the math in my head) seems to be roughly the same as the time you are together... he may be doing to another woman the same as he is doing you.
The whole scenario doesn't feel right to me, and I know it doesn't feel right to you, so take steps to protect yourself from the embarrassment/anger/abandonment/shock/disgust, and whatever else you feel when you are trapped in his vortex. And please, hold off on having a baby until this has shaken out. No sense exposing another person to the madness - make sure your kiddo has a stable home!
I wish you well. You are stuck in a very delicate dance right now, and I urge you to get some help to change the tune. If you are satisfied with the response, please hit "Accept." That is the only way I can receive credit for my answer. Thanks-