Not gaining weight

Hello everyone so I went to the dr. Yesterday and they were show that I only gained a few oz. pre pregnancy weight was 180lbs last month I was 172.8 lbs. and yesterday being now 16weeks I'm 173.0 lbs is this normal ? They were worried that I wasn't gaining weight but will continue to monitor because my first daughter was a premie baby as doctors told me she had no room to grow  any advice or has anyone gone through something similar ?