Most reliable criteria for gestational age estimation in 1st trimester ultrasound is:

A i.e. CRLRef: Textbook of Obstetrics by Sheila Balakrishnan, 2nd edition, page 476Explanation:First trimester scan is also called Dating scan.Crown rump length is measured from the top of the head to the bottom of the buttocks excluding limbs.CRL between 9 and 11 weeks is the most accurate method of dating.Error is about +1-5 days.CRL in cm +6.5 gives a rough estimate of gestational age in weeks.Useful prior to amniocentesis and chorionic villous sampling and in diagnosis of fetal growth restriction.