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.
In very early pregnancy, the embryo and pregnancy sac may simply be too small to see very much at all.
But with every day which passes, the embryo becomes bigger and more advanced in its development.
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.
Generally, ultrasounds which are performed in the first trimester are within 3-5 days of being accurate in terms of assessing gestational age.
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.
So obtaining accurate measurements and visualising them clearly on the screen is a little easier than when they are able to do somersaults and move around a lot. This is also more likely if you are having a transvaginal ultrasound. One of the reasons for this is because the heart needs to work harder and more efficiently to pump oxygenated blood around its much larger body and brain.
The quality of the equipment and the skill of the sonographer are very important when doing any pregnancy ultrasound.