At 9 weeks, approximate CRL in mm of a fetus would be:

Crown rump length(CRL) is measured from the top of the head to the bottom of buttocks excluding limbs.CRL between 9 and 11 weeks is the most accurate method of dating.Error is about +/- 5 days.CRL in cm +6.5 gives a rough estimate of gestational age in weeks.Uesful prior to amniocentesis and chorion sampling and to confirm later intrauterine growth restriction. Refer page no 476 of Text book of obstetrics sheila balakrishnan 2 nd edition.