What is more accurate in dating day of conception, going 14 days after the last period, or the 7 week ultrasound dating?

Depends. If normal pregnancy, 7 week ultrasound should be consistent (+/- 3 days) with estimated gestation by last menstrual period (assumes ovulation 14 days after lmp). This may be modified if pregnancy occurs after infertility treatment which can alter menstrual cycle. In such cases of insemination or ivf, dating may be more accurately calculated based on ovulation.