Weight Gain

Hello mamas! :)
I'm just curious when you all started gaining weight (if any) during your pregnancy. I'm 16w5d with my second and the past three appointments I've weighed the exact same. My doctor isn't concerned because I haven't been sick or anything so I know I'm okay. I just think it's crazy that I haven't gained any! I told my husband that it's because I'm loosing my butt and that weight is going to my stomach! 😂