Now that I have more hindsight, my best advice is to have at least one or two or 10 back up plans for yourself and your child. Always make sure you have a way to stay safe financially, physically and emotionally should you need to protect the both of you.
Remain realistic about your situation. Don't try to hold onto his potential as the reason for staying. Your future is now. What your dealing with is the present. Holding onto what could be doesn't help your here and now.
Pay attention to your self talk because that more than anything I think keeps us stuck in situations and feeling helpless and lost. The most unkind person to us is most often ourselves.
If in your mind somewhere, you feel you can change him, give that notion up. The only person that can help him is himself. Any need to change within him will only come through his own motivations and reasons, not because you or your child needs it. Also, if you provide for his every need, he has no incentive to feel the impact of the consequences so don't step in to save him ever. Your only concern is to care for you and your child.
Don't isolate yourself. Get help wherever you can and accept help offered to you. Use every tool out there available to help you stay focused and deal with your emotions. Start looking now for places and people that can help you now and later. Don't wait until you need something.
Go to Narc-Anon or Ala-non and counseling. You need to be surrounded by support. If he doesn't get clean fast or even if he does, your going to feel alone during your pregnancy so try to prepare for that now. If he doesn't get clean, he'll be entrenched in his own needs seeking a fix and if he gets clean, he will need to dedicate most of his energy to participating in his recovery so he can be well for all of you later. So either way, expect that despite your own needs, he's going to appear selfish throughout your pregnancy. No momma or momma to be should have to feel alone so seek support other than him.
I'm so sorry you are going through this. My prayers and best wishes for all of you