Explain to them beforehand, what a baby is.
For example (I did this with my daughter, who was 3 almost 4, when I had my 2nd child. ):
-a baby cries/wakes/I nurse her
- babies cry a lot because they cannot talk yet, like you can
- Mommy nurses her
- I will wake up to take care of her at night. You don't have to wake up. ONLY Mommy
- Daddy will help you too
- babies are very delicate, need to be gentle (practice with a doll)
- every month a baby changes. I'd explain to my daughter how. Motor-skills, cognition, etc.
I explained to my daughter, BEFORE I had the baby, what will happen and when her baby brother came home from the hospital.
I napped, with my daughter when I was pregnant. Explaining: Mommy needs to take care of her tummy, I will get tired the bigger my tummy gets etc. She understood like a champ.
I made my pregnancy about prepping her. It was not 'my' pregnancy... but my time to prep my daughter about it and for when baby brother came home.
My daughter adjusted very well. She knew what to expect. She knew how I was still there for her, even with another baby. Because I explained everything to her, before, I brought home my 2nd child.
And once home, I spent a ton of time, talking with her and explaining everything to her. Or any questions she had. Thus, she 'knew' what a baby was, how they are, what they do, and why. And what Mommy does. Too. And how, she is, "always my first baby." I incorporated her into everything, with me. For as much as she could understand for her age and her maturity. I spent my whole pregnancy, prepping my daughter about it. Even going with me to my prenatal appointments, which my Doc encouraged and he taught her how to use the Doppler heart monitor on my tummy. And of course, took tons of photos with her and my growing tummy, each month. She'd sing and talk to my tummy too, her baby brother in my tummy. She already bonded with him, before he was born.