Re: How can a doctor tell how far along u are?
They measure by ultrasound, and how they do that is they draw a line on the screen and measure the fetus/baby head to rump and it gives them a reading for the exact week and day it measures. Also, if you know your LMP you can determine how far along you are. I was unsure by a week or so, so I got a "dating ultrasound" and it determined I was 8 weeks at the time. I hope that answers your questions.