How do doctors use ultrasound to estimate a due date in pregnancy?

How do doctors use ultrasound to estimate a due date in pregnancy?

Kenneth W. Chin (M.D., F.A.C.R.) gives expert video advice on: How does ultrasound work?; Is ultrasound safe?; Why has ultrasound been controversial? and more...

Doctors use ultrasound to determine a due date on the basis of the size of the fetus and the size of various parts of the body, such as the size of the baby's head, or their abdomen, or their extremeties.