With DS2 and DD I lost weight right up until quite late on, and then I only put on what I had lost. They were both quite dainty but perfectly healthy.
This time, I have put on weight but not an unhealthy amount, although I did put on loads really quickly recently then lost it all within a week (just more than a stone) because it was all water! This was down to having really low potassium. Sometimes it can make you retain water and this had forced my legs from my toes to my hips, swell up hugely.
Anyway, I wouldn’t worry too much. As long as everything is all normal and you’re not concerned, some people do just lose weight rather than put it on. If you are worried then speak to your midwife, perhaps they can arrange for a further scan to ensure all is fine to help put your mind at ease