What is a dating ultrasound
The GS is not identifiable until approximately 4 1/2 weeks with a transvaginal scan.Gestational sac size should be determined by measuring the mean of three diameters.
The rhombencephalon of the developing brain is visible as a prominent fluid space posteriorly.
This is a risk assessment based on age, heritage,history and a specific ultrasound measurement at the back of the fetus neck.
The accuracy of this is increased by factoring in the levels of b HCG and Papp A in the maternal blood.
Failure to identify (with transvaginal ultrasound) a yolk sac when the gestational sac has grown to 12 mm is also usually indicative of a failed pregnancy.
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.
This is the result of normal midgut proliferation and will resolve by 11 weeks as the fetus lengthens.