Accuracy 1st trimester ultrasound dating
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.
The first trimester is defined as the first 13 weeks of pregnancy following the last normal menstrual period (some authors refer to early pregnancy as 0-10 weeks).
It can be divided into a number of phases, each of which has typical clinical issues.
As the baby gets bigger, it starts to express its individual growth potential.
The size of the baby correlates less and less with its age as time goes on.
For transvaginal ultrasound you can have an empty bladder Dating scans are usually recommended if there is doubt about the validity of the last menstrual period.
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.
) 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).
the baby and its heart beat can be detected relatively easily with trans-abdominal and trans-vaginal examination.
These phases are: During the first trimester, the pregnancy progresses from a tiny gestational sac with no visible embryo, to an ~80 mm fetus with identifiable features and internal organs.