How does scan calculate pregnancy?

Question

My LMP date and the week the scan showed are slightly different and im a bit confused. How does the sonography actually decide how many weeks pregnant you are? Trying to understand which date to go by.

Antara Rana · Answer

The doctor just measured the baby on the screen and told me how many weeks. So clever na, I was amazed. She also gave me the due date from it.

Kavya Walia · Answer

I had calculated my weeks from my last period date in an app, but the scan adjusted it slightly. Apparently the early scan is more precise. Both were close though.

Zenia Chavan · Answer

My doctor explained the early scan measures the baby's size, like the crown rump length, and uses that to estimate weeks. Later it's based on head and limb measurements. The first trimester dating scan is the most accurate she said.