DD1 was 3.5 when I got pregnant wit DD2.
I felt like utter shit from about six weeks, and spent a lot of time lying on the sofa so we made a decision to tell DD1 fairly quickly so she knew why!
I bought a couple of books (including 'There's a House inside my Mummy' and one called 'I'm a big sister').
Then, one evening, when we went up to have stories at bedtime, I told her we had a special thing to tell her She was going to be a big sister. It would be a very long time away, but at the moment sometimes it made Mummy feel a bit poorly. When we had scan photos we showed her, and got out her old scan photos to show her and compare.
And then we read 'There's a house inside my mummy' which even talks about morning sickness!!
We didn't do overkill - maybe once a week or so we'd read one or other of the books. 'I'm a big sister' looks at all the things big kids can do that babies can't (like eat ice cream!) so it makes being the big one a positive thing.
We always talked about her being a big sister rather than us having a new baby as I felt that might give her a role, rather than making her feel pushed out by a new model. And I talked about all the things she'd be able to help me with because she would be big enough and the baby would be too small to do much.
As we got nearer to the time, she chose something to give the baby, and after DD2 was born and DD1 came to the hospital to see her, I made sure the baby was in the crib so I could give DD1 a cuddle without the baby in the way, and when she looked in the crib, the baby had a present for her....
It largely seems to have worked. She adores DD2 (they are now 5 and 18 months). The only real problem I have is when DD2 trashes the lego construction/train tracks/happyland village carefully laid out!
Congratulations on your pregnancy.