Just make sure you're not in any pain. At 18 weeks I was measuring 27 weeks (singleton size) and felt a lot of "stretching" on the right side of my abdomen and my belly was tight as a drum. I went in for an US and they found the fluid levels were off. Baby A had 11 cm of fluid and Baby B had 2 cm. I was diagnosed TTTS and had to have surgery. All turned out well and both babies are healthy, fluid levels returned to normal and the pain in my belly went away and my bump reduced in size a bit. My Dr said every woman measures differently depending on body type, but you shouldn't feel any unusual pain. I am sure you're in fantastic hands, but dont be afraid to ask if the fluid levels are ok for the babes. My OB/GYN assured me at 8 weeks I would be crazy to worry about TTTS and at 18 weeks it was a confirmed diagnosis by an MFM specialist. Needless to say, my OB felt like crap afterward and has since learned to listen to his patients a little more. Good luck, and keep us all posted!