Because few women know the exact day they ovulated or conceived, an ultrasound done in the first trimester of pregnancy has been shown to the be the most accurate way to date a pregnancy. If an ultrasound date in the first trimester differs from your LMP date by seven days or more, we would go with the ultrasound.

Do you go by last period or conception?

Pregnancy is counted from the first day of the womans last period, not the date of conception which generally occurs two weeks later.

How accurate is dating pregnancy from LMP?

It has been reported that approximately one half of women accurately recall their LMP 2 3 4. In one study, 40% of the women randomized to receive first-trimester ultrasonography had their EDD adjusted because of a discrepancy of more than 5 days between ultrasound dating and LMP dating 5.