Is my bra size my nursing bra size or is it different? UK?

Im 34dd in my nursing bra but want to buy this for a backless dress a href="http://www.asos.com/Mel-B/Ultimo/Ultimo-A-D-Miracle-Solutions-Gel-Filled-Backless-Body/Prod/pgeproduct.aspx?iid=146656&SearchQuery=backless&sh=0&pge=0&pgesize=20&sort=-1&clr=Black" rel="nofollow"http://www.asos.com/Mel-B/Ultimo/Ultimo-…/a I was a 32d before pregany and dont feel Ive changed that much HELP??