These differences rarely effect gestational age dating by more than a day or two.
The very early embryonic heart will be a subtle flicker.
In these cases, one of the twins fails to grow and thrive.
Instead, its development arrests and it is reabsorbed, with no evidence at delivery of the twin pregnancy.
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.
The Crown Rump Length (CRL) measurement in a 6 week gestation.
A mass of fetal cells, separate from the yolk sac, first becomes apparent on transvaginal ultrasound just after the 6th week of gestation. The fetal pole grows at a rate of about 1 mm a day, starting at the 6th week of gestational age.
Thus, a simple way to "date" an early pregnancy is to add the length of the fetus (in mm) to 6 weeks.
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.
The CRL is a reproducible and accurate method for measuring and dating a fetus.
Early ultrasonographers used this term (CRL) because early fetuses also adopted the sitting in the chair posture in early pregnancy.
Sometimes there is difficulty distinguishing between the maternal pulse and fetal heart beat.