Sorry I'll be 20 weeks on Monday
You can buy clothes at any time, if you have plenty space. If you are planning a baby shower, I'd wait till you see what other ppl give you, then you can go about getting what is still needed. Got newborns, you don't need much, and I advise getting different sizes of clothes. I'm also about to be 20 weeks, but with my third child. :) good luck and happy parenting.