Hi, there are norms for aortic size, which depends upon the body size of the patient (body surface area (BSA) takes into account both height and weight). If the measurement of the aorta on echocardiography exceeds the upper limit of normal for the patient’s body size, then it is considered to be enlarged. This can be seen in a variety of congenital heart conditions, most commonly a two-leaflet (bicuspid) aortic valve. It can also occur due to connective tissue disorders which include syndromes such as Marfan syndrome, Loeys-Dietz syndrome, and Ehlers-Danlos syndrome. These are inherited conditions passed from generation to generation. Surgery is indicated when the aortic enlargement is severe. So, please discuss with your treating doctor. Regards.
As Dr. Anitha mentioned, normal aorta size depends on a person's overall body size and not on age. The doctors have a formula for height and weight that generates the expected aorta size. I don't know what diameter your daughter's aorta should be, but 3.1cm is about the size that a grown woman's aortic root should be. (The aortic root is the part of the aorta that can be measured by echocardiography.) A Z-score of 4.9 is pretty big. The Z-score is a type of comparative measurement that uses the population standard deviation as a yardstick. 4.9 standard deviation units is quite different from expectation.
You asked if there are any other tests that can measure aorta size, and both CT scan and MRI can do that. CT is usually considered to be the most accurate way, the "gold standard," for measuring the size of the aorta. I assume the doctors will do a CT scan prior to performing surgery on your daughter. I'm guessing that the reason they have not done it yet is to avoid exposing your daughter to the relatively high dose of ionizing radiation that is involved. Since she has this condition at such a young age, she is a candidate to have a number of CT scans over the course of her lifetime, so it does make sense, at least to me, to use echocardiography until it is closer to the time of surgery.
I am sorry your daughter has to go through something like this. I hope the surgery is still a long time off. With a skilled surgeon and team, the outcomes are good.