The measurements are probably pretty accurate, but I would think that it is hard to be accurate with the weight. I have also read that after 38 (or maybe 36?) weeks that babies are full grown and just spend the last few weeks laying down fat and to me, how much a baby weighs is more to do with how much fat they have, than how big their head or arms or torso is KWIM? This is all my own thoughts btw, I am no expert in these things! But for example, I am roughly the same height as my sister and I think our heads are probably the same size, but I would easily weigh 20kg more than she does - so if someone just measured us without actually being able to see us, how will they be able to measure the differences in weight?