Early ultrasound weeks off dating
Using a transvaginal approach the fetal heart beat can be seen flickering before the fetal pole is even identified. It may be below 100 beats per minute but this will increase to between 120- 180 beats per minute by 7 weeks.
In the early scans at 5-6 weeks just visualising a heart beating is the important thing.
Thus, a simple way to "date" an early pregnancy is to add the length of the fetus (in mm) to 6 weeks.
The earlier in pregnancy a scan is performed, the more accurate the age assignment from crown rump length.
The average sac diameter is determined by measuring the length,width and height then dividing by 3 .
The gestational sac(GS) is the earliest sonographic finding in pregnancy.
The very early embryonic heart will be a subtle flicker.
This may be measured using M-Mode(avoid Doppler in the first trimester due to risks of bioeffects). Compare to the maternal heart rate to confirm that you are not seeing an arteriole.
An ectopic pregnancy will appear the same but it will not be within the endometrial cavity.