Pre preg I'm about a 10, sometimes an 8, so allowing for breastfeeding breasts and a bit of extra flesh, I think I owned one size 12 and maybe one size 10? They're pretty stretchy, so you'd really just want to base it around your breast size. I had one second hand and one new, and I wore the new one during the day, but not out. Probably as my milk settled I was wanting to wear bras so I could wear nicer 'I've actually made some effort' clothes.