These scans are only used to estimate the due date of your baby if this is all you have to go on.
If you have more than one ultrasound during your pregnancy, giving you 'multiple dates', then the earliest ultrasound estimate should be used, because it will be more accurate.
Correlation between GA determined by ultrasound and ovulation day was excellent (maximum difference 10 days); however, pregnancies dated by ultrasound were 3 days advanced.
The difference between LMP estimates and estimates based on ovulation day or ultrasound was 9 and 12 days, respectively.
A uniform rise in h CG on each day of pregnancy was seen using all reference methods.
The accuracy of h CG measurement in determining the week since conception was more than 93%.
This is presuming that the pregnancy is actually at this stage of development.
However, the accuracy of the ultrasound examination is always dependent on the skill of the sonographer and the quality of the equipment.
Usually the expected date of delivery (EDD or EDC) is calculated from your last menstrual period - if the early dating scan calculates the EDD to be within 5 days of the EDD from your last menstrual period.
Female volunteers seeking to conceive (at 5 US sites) collected daily early-morning urine for up to 3 menstrual cycles. Conception cycle urine was quantitatively assessed for luteinizing hormone and h CG.
Summary statistics for GA using each reference method were determined (n = 131).
The size of the baby correlates less and less with its age as time goes on.