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.

Is LMP or ultrasound more accurate?

When ultrasound was used instead of certain LMP, the number of postterm pregnancies decreased from 10.3% to 2.7% (P <. 001). Conclusion: Ultrasound was more accurate than LMP in dating, and when it was used the number of postterm pregnancies decreased.

Can your LMP be wrong?

Clinical determination of EDD, 280 days after the last menstrual period (LMP) still plays a role but may not always be accurate due to variability in length of an individual womans cycle length or timing of ovulation.