Menstrual Period (LMP)
- Mean Sac Diameter =
average diameter of gestational sac measured in 3 planes
- Crown Rump Length (CRL)
= longest measurable length of the embryo or fetus excluding the
lower limbs, yolk sac and umbilical cord.
- Crown (middle of
midbrain) to embryonic or fetal rump.
- From 6-8.5 menstrual weeks
the maximum length of the embryo is not accurately represented by the
CRL as normal flexion of the embryo results in the middle of the
midbrain being located below the highest point of the head. As the
pregnancy progresses, embryonic head extension makes the middle of the
midbrain the highest point.
Best measurement is greatest length and not CRL.
S. Accuracy of gestational age estimation estimation using fetal
crown-rump measurements. Am J Obstet Gynecol 1993;168:903-908.
S. Fetal biometry: Clinical, pathological and technical considerations.
Obstetrical and Gynecological Survey 2001;56(3):159-167.