It was the tape measure on the tummy, and i feel sooo informed now this all makes soo mich sense! Thank you ladies :)
It also depends on how your ob measured. If it was via fundal height (tape measure on your tummy), those are known to be very inaccurate measurements.
I'm 28wks now but at 18 wks baby was measuring to 19 almost 20 weeks. They said that they will not change the due date because after 12 wks all babies grow to different sizes. no really standard after that.
Babies vary so much in size as they come towards the end of the pregnancy. All you doc is telling you is that you baba is a little bigger than the average baby. He won't change your due date as the early scan are the early scans are the most accurate as early on babies don't differ too much in size. The most accurate due date is the one they set with your very first scan. Hope that helps explain things a little clearer. So excited...not long to go now! Xx