I’m so sorry @Headhurts. I don’t want to act like there’s one good answer. In short you should of course do what you think is right and what makes you most comfortable. I would imagine we all put ourselves in your position and answer from that way of thinking, not necessarily from your situation.
That said, as a person who has been with someone through several failed pregnancies, I’d always want to know. I’d want to know about the pregnancy, but more than that I’d want to know so I could help you however you needed. There’s nothing that can be done about the pregnancy, but the other people involved are still there and can still help one another. You can help him through whatever his reaction is as well.
To me the alternative is you’re going to be upset and at least a little stressed, and then you’re going to add to that by not letting someone who sees you every day know why. If he knows you very well, he’ll know something is up, he’ll know you’re not telling him, and he’ll have to fill in the blanks.
That seems worse than sharing the truth and helping each other with it.
Good luck with whatever you decide, I hope the appointment tomorrow is just confirmation of everything you already know and goes well.