13lbs. I lost a stone upto about 20 weeks due to lack of appetite and have regained that plus another 13lbs. I know my weight gain exactly and when I gained it, as I am weighed whenever I go to consultant clinic/scan (never when I go to midwife), so it's not just guesswork. I don't feel any fatter than I did at 20 weeks when I was nearly 2 stone lighter than I am now, so if I didn't weigh myself, I'd assume I was still a stone lighter than pre-pregnancy.
As I am 13lbs up on pre-pregnancy and the estimated weight of my baby from scan is nearly 7lbs, I think a few days after birth I will be slightly less than pre-pregnancy.