My baby's estimated weight at 28wks was 3lb, which is in the 50th percentile so I'd say it's a good weight. I wouldn't worry too much. The other thing is babies grow at different rates and have growth spurts so it may be that when you go back, weight may have slowed or may slow later on. And bear in mind it is only estimated, they can't know for sure how much any baby weighs until it arrives, scans can be out by anything up to like 8oz either way :)
Or this App I think says how much babies gain per week toward the end if you look ahead a bit.
Google's prob your best bet here.