Measurement of embryo in gestational sac

Image of an early gestational sac demonstrating the early embryo. Calipers are placed at both ends of the embryo measuring the longest length from the "crown to the rump," giving the crown-rump length. This measurement is used for dating the pregnancy. Surrounding the embryo is the developing amnion as shown by the hyperechoic circular line. Note how the amnion is the approximate size of CRL at this gestational age of about 7.5 weeks.