Oh that sounds tough. I hope things are ok for you? Doesn't sound good if you're in hospital.
I would give simple information close to the time. At this age they are generally not good with understanding the passage of time, so tell her a short time before you go in, and say you'll see her soon, and so forth. And then prepare her for each parting and each reunion. She'll get used to it pretty quickly, though she's likely to be upset every time she says goodbye (and maybe when she sees you again, too, as she's relieved to see you).
Depending on her personality and ability to communicate, your DH could frame it as an adventure each time, or prepare by doing pictures/gifts for mum...Always reassuring her that mum will be coming back again in time. If she has questions, answer them as honestly as you can, but simply. They don't need a lot of information, often, just the basics. So long as it is sincere.
All the best.