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When the EDD based on ultrasound is more than 8% different than the EDD based on the LMP, then the estimated due date based on ultrasound is generally used and considered preferable.
The later the ultrasound is performed during the pregnancy, the greater the margin of error in the due date.
Although it is always 8%, the actual error is greater because 8% of six weeks is a smaller number of days than 8% of 36 weeks.
Let’s say a woman comes in for an early pregnancy ultrasound and she is 8.1 weeks by LMP according to a pregnancy calculator.
Other early symptoms include urinary frequency and nausea.
Many women describe nipple tingling as their first symptom.
A fetal pole with cardiac pulsation can be detected at about 5.5 weeks.
A pregnancy test can be positive usually no earlier than 3.2 weeks, if even then.
Some women who do normally have a 28 day cycle may have other reasons for making the LMP inaccurate; for example, if they were taking birth control at the time of conception or just don’t remember the exact date.So since the difference is greater than 8%, then the EDD is changed based on the ultrasound.