Okay, speaking generally, measuring head growth:
Head circumference (also known as OFC for occipito frontal circumference) is a pretty crude measure. It is used as a proxy for brain growth, as it is the growth of the contents of the skull (usually the brain) that mainly govern the growth of the skull. This is not about the growth of the entire head, but of the vault of the skull, so you're basically measuring from the forehead and the front to the most prominent bit at the back of the head, and back again. No ears in the way, no face in the way, preferably no hair/hats/tape holding lines in the way. It's not especially accurate in most small children because they move. In some ways it would be easier to measure in a child who cannot move. If the head circumference is not increasing appropriately, it is commonly because the brain is not growing enough. It is occasionally because some of the skull bones have joined together too early, but this tends to give a very noticeable unusual head shape, and is pretty easily diagnosed. Sometimes the head circumference goes up too quickly, and that can be that the brain is growing fast, but that would be very rare. Much more commonly, it is because the fluid spaces in the brain are expanding, increasing the pressure in the brain and skull. Oedema over the skull can also cause over reading.
It is recognised that OFC measurement is subject to a fair bit of inter and intra user variability (2 people don't get exactly the same number, one person doesn't get exactly the same number twice). There are 2 safeguards against this. The first is that you monitor over a time period, and like with baby weights, it is the general pattern of increase you look at, not one absolute value. The second safeguard is that most paediatricians have been trained not to measure the head circumference once, but to each time measure it 3 times, to make sure you're getting something fairly consistent, and usually use the largest good measurement.
If a baby/small child's head circumference is growing more slowly (or indeed more rapidly) than would be expected, the first test is usually for the measurement to be repeated. If, however, the general trend is abnormal, then that is when you move onto scanning the head. In really small babies, the first scan is often USS (via the soft spot) - it's quick, it spots big problems, no radiation, but not great detail. CT scan is relatively quick, but radiation, and medium level detail. Tends to only be done in relative emergency (or for a few rare things...this'll get too complicated) because of the radiation. MRI gets the best brain detail, but takes longer. There's no radiation, but nothing ferromagnetic can go in the scan room, so real palaver to perform on unstable PICU patients - different kit for almost everything.
None of the scans give you the head circumference. You could technically try to reconstruct one from all the pictures of slices of the head, but not something anyone ever really tries to do. Because you're using it to look directly at the brain, not to try to reconstruct a measurement whose main purpose is to tell you whether to scan a brain.
Hope that helps.