Oh and with Ky I always measured spot on "Normal" - i.e 20cm at 20 weeks and so on. He was 6lb 9.5 oz - So classed as "Under average Birth weight".
I think the average for a boy at birth is 7lb 11oz.....Its not 100% accurate, but gives a general guide to your health care provider as to whether your baby is particularly big or small. Generally if they suspect a problem either way, a scan will be used to confirm as the position of the baby can affect the fundle heights accuracy.