I don't think there is a problem - it is quite likely that this bub will be smaller than the others and I have known a lot of women who have had pretty decent size babies and then have one that is quite small. It's just the luck of the draw I suppose. Even with my own pgs I knew that DD1 wasn't going to be very big because I wasn't that big at all. With her my belly was nice and round and average size, but with the other three I was massive, with a great big pointy belly. But if you are worried maybe you should try to have a scan just to make sure growth isn't compromised.