I agree that it's best to be proactive at this point.
I would tell your boss that you are in the process of adopting, and briefly explain what that wait looks like (could be next week, could be next year). Emphasize that you like working there, and want to continue in some capacity. That you know FMLA won't apply, but want to discuss options for after a baby gets here, whether that's next week or next year. Have some ideas going in so your boss isn't hit with the news AND has to scramble to figure out an option on the spot.
Yes, it may be an uncomfortable conversation. Or it may go very smoothly. But IMO it's better now than later.
FWIW, our match was with a baby already born. We got The Call on a Thursday morning and took DD home Friday night. You never know what situation is going to come up