You don't want to make them too soon because the elastic might give out by then. 5 years is a long time!
I know it's tempting, but wait for that positive pregnancy test! Within 3 weeks of my + I had already made 24 XS Very Baby fitteds for my baby. (Of course now I'm regretting making so many because I don't think they are absorbent enough and I'm trying to sell some with no luck; might need to just keep them and use doublers...) But i ramble....
You could make a couple of newborn diapers just for fun though, or to swap with someone here.

You could make a couple of newborn diapers just for fun though, or to swap with someone here.