What is more accurate in dating conception, going 14 days after the last period, or the ultrasound crown rump?

CRL!!! Due to biological variability in ovulation and implantation/fertilization timing, the use of crl by first trimester ultrasound is more accurate than use of last known menstrual period as dating criterion. In fact, studies have proven that even with certain menstrual dates, the sonographically-determined due date before 20-24 weeks is more accurate for clinical purposes (see jason gardosi's papers).