You're worried about looking after any baby let alone one with health complications.
It is worrying, but let me tell you my story.
I was 17 when I got pregnant. I was terrified but I had a perfect little boy and the parenting thing just happened. You learn as you go.
When DS was 4 he had a horrible severe brain regression, lost all his skills and developed some health problems and disabilities, plus severe challenging behaviour. At that time I was 21/22 and a single mother.
DS has autism now caused by the brain regression, a heart murmur, seizures, Reynauds, high arches, double incontinence, severe learning disabilities, just to name a few.
The cause of the initial regression and subsequent regressions is yet unknown, they think it's a rare genetic disorder, but one they have not found and named yet. So he is going into research studies. They wouldn't have found this on any prenatal tests.
DS is 8 now, the house is set up for him, I've fought for him to go to the best special schools available here, I do everything I can, on my own and I don't even feel that exhausted by it most days.
The thing is, that you just do it, you just deal with everything, from the usual weaning, night feeds and toilet training, to seizures and special needs.
As long as you have love. Which is sounds like you have in spades for a baby that isn't even here yet.
It's pointless stressing about the unknown, just enjoy the now. You are pregnant with a gorgeous little baby and you are going to get gorgeous baby cuddles!