In some respects the seven week ultrasound can be quite accurate because the embryo is developing very quickly.They are also not as mobile and active as they will be with future development.This is the period of time which provides the most accurate assessment.Generally, ultrasounds which are performed in the first trimester are within 3-5 days of being accurate in terms of assessing gestational age.
At seven weeks of gestation it is still too early to identify what gender (sex) the baby will be.
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.