Even average babies can differ by up to 2 to 3 "weeks of growth";.
These scans are only used to estimate the due date of your baby if this is all you have to go on.
) the fetus is clearly seen on trans-vaginal ultrasound and the heart beat can be seen at this early stage (90 to 110 beats per minute under 6 to 7 weeks, then 110 to 200 beats per minute as the baby matures).
Ultrasound examinations from 12 to 22 weeks are regarded as being within 10 days of accuracy (or up to 10 days earlier or 10 days later than the woman's calculated due date).
Ultrasounds performed after 22 weeks gestation cannot be used to estimate the due date of the baby because the size no longer reflects the age very well.
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.
The EDD from the early dating scan is used - if the last menstrual period is not known or is unreliable, or the dating scan differs from the last menstrual period dating by more than 5 days.
This is presuming that the pregnancy is actually at this stage of development.
Sometimes a trans-vaginal examination shows that your pregnancy is less advanced than expected.Also the fetus cannot bend and twist too much yet, so the length of the body is fairly constant.However, the accuracy of the ultrasound examination is always dependent on the skill of the sonographer and the quality of the equipment.The most accurate time is between 8 and 11 weeks gestation.This is because the fetus is growing so quickly that there is a big difference in size from week to week.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.