You are probably losing fat even though you are gaining baby weight, if that makes sense?
Your carers wouls be able to tell if the baby wasnt growing (or they had concerns) and Im sure would not hesitate to send you in to be checked out if they were at all worried.

I've known a lot of people to stay the same or lose weight during their pg. One lady I know (who was quite over weight at the beginning of her pg) lost over 15 kilos in her pg, then another 15 or so after the baby and was almost the weight she was in high school after about 3 weeks.