A 23-year-old female asked:
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when 1st trimester ultrasounds date pregnancy how do they base the gestational age? do they assume conception occured on cd14?

1 doctor answer
Dr. Gerard Honore
28 years experience Fertility Medicine
US: First trimester ultrasound is based on biometric measurements only, the embryonic crown-rump length, which is extremely reliable when done by a skilled sonographer. No assumptiosn need be made at all. The ega is however reported correctly, by convention, from the first day of a hypotehtical last menstrual period; so in that regard it 'assumes' conception on cd 14.
Answered on Feb 20, 2014
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