Earlier ultrasounds are more accurate in terms of predicting the due date, so thats why doctors will usually use the dates and measurements from the first ultrasound of the pregnancy as a reference. As pregnancy progresses, the accuracy of an ultrasound for predicting due dates decreases.

Which date is correct LMP or ultrasound?

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.

Why is pregnancy calculated from date of last period?

If you were having regular periods before pregnancy, your doctor will calculate your due date based off of your last menstrual period. This goes back to the fact that in order to get pregnant, your body ovulated—or released an egg—roughly in the middle of your cycle and it was fertilized by sperm.

Does pregnancy start from first day of last period?

Your weeks of pregnancy are dated from the first day of your last period. This means that in the first 2 weeks or so, you arent actually pregnant – your body is preparing for ovulation (releasing an egg from one of your ovaries) as usual.