You buy them when you need them. My friend needed to rethink her wardrobe from about 3.5 months as she looked like she had been inflated with a bike pump. I was 6 months and then expanded very rapidly with DS 1 and about 5 with DS2
I'm 15 weeks. My summer dresses (that I wear for work and smartish) are fine and I hope to get through to autumn without new stuff.
At the weekend I bought some waist expander bump bands from Jojo as my weekend jeans and shorts are starting to feel a tad tight. I'm hoping tops will be ok....
I've already started buying maternity dresses for the autumn. I got a few in next and just ordered 5 in the sale from seraphine as a lot of what is obviously last years winter stuff is on sale!! I like planning ahead. I just know if I leave it until I absolutely need it I won't be able to find anything I like in my size.