Why are ultraound edd more accurate than your lmp edd?

They are not: Ultrasound based edd were established by doing ultrasound measurements on women who absolutely knew when they conceived. Thus they are just as accurate as a very accurate lmp. Some women do not remember an accurate lmp.

Pregnancy dating: Lmp pregnancy dating relies on ovulation being on day 14, but some if not many women ovulate a few days before or after. First trimester ultra sound dating, especially first 8 weeks, is much more accurate by measuring the length of the baby correlating with the appearance of the heart beat.

Not always true : Early in fetal life all babies grow at the same rate. The longer into the pregnancy the more each baby grows slightly differently. Therefore, early in pregnnacy we can measure the size of a baby and very accurately predict the due date. Later on though, the sonographic measurements less accurately predict the due date.

Good Question: An LMP (last menstrual period) edd (pregnancy due date) is the most accurate if everything is normal and ovulation occured 14-16 days after the lmp. Unfortunately, abnormal or delayed ovulation or missed ovulation are causes of inaccurate edd predicting. An ultrasound done in the first trimester of pregnancy will most accurately validate the LMP edd or invalidate it, hence the scan edd.

They aren't.: Determining due date is not exact regardless of the method used. We are looking for the best estimate. A sonogram early in the pregnancy (8-10 wks) is an accurate method of dating a pregnancy, but still has a margin of error of several days. Later sonograms are less accurate. If the LMP edd is within the margin of error of the sonogram, the LMP date is the best estimate.