At around seven weeks of pregnancy, it’s reasonably common for women to have their first ultrasound.
This can be for many reasons but the most common is to confirm pregnancy and to check that the embryo is viable and “everything’s ok”.
This is because although every little embryo is unique, there are still defining characteristics of development which apply to all embryos at particular stages of early gestational development.
As the embryo matures to a foetus, individual genetics and growth factors influence its growth.
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.
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”.
This is why an early pregnancy dating scan, rather than one done in the later stages of pregnancy, is considered more accurate when assessing the expected date of delivery.
With maturity, the size of the baby correlates less to its age than in the early weeks.