Ultrasound uses the size of the fetus to determine the gestational age (the time elapsed since the the first day of the last menstrual period). The accuracy of the ultrasound estimate of the gestational age varies according to the gestational age.
Also that he had never seen anything like this before in his 15 years.... While my di/di twins measured a few days apart early on, my dr told me that they like to see them measure within a week of each other, that might be the 'benchmark for difference' that your OB is referring to that you're just barely on the outside of. Think about the margin of error for something like this- all throughout my pregnancy, every time anything was measured, I was wondering how accurate it was. The fact that you have 2 heartbeats is a very good sign!Use of the LMP to establish the due date may overestimate the duration of the pregnancy, and can be subject to an error of more than 2 weeks [5-7].