How does the doctor tell how old your fetus is?

Marie

New member
I'm pregnant, and from my first day of my last menstrual pd I'm 17wks & 2dys pregnant. But i had an ultrasound done 2 days ago. And the ultrasound tech told me i was only about 13 to 14 wks but she couldn't give me an exact age of the fetus. She said my doctor would be able to tell me though... I'm curious about how the doctor can exactly tell me the age of my fetus.?
 
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