Ultrasound for dating
The limbs and the yolk sac, though obviously important, are not the primary means of measuring growth.
An average length of the embryo at 7 weeks is anywhere between 5mm-12mm. Obviously, every pregnancy is unique and individual factors influence the size of the embryo at this early stage, and the embryo shows development week by week.
A duration of only a week can make a significant difference in terms of what can and can’t be seen.
At seven weeks of gestation it is still too early to identify what gender (sex) the baby will be.
It is also impossible to do a thorough foetal screening assessment because it is still just too premature in terms of embryonic development.
However, general “mass” structures such as a head and body can generally be seen in the embryo at seven weeks.
With maturity, the size of the baby correlates less to its age than in the early weeks.Other reasons to have a 7 week ultrasound are to: The embryo will be measured from the top of its head, the “crown” to its bottom or “rump”.