Mean head circumference at birth is
Head circumference:-

 Measured with a nonstretchable tape passing through the maximum point of occipital protuberance posteriorly and at a point just above the glabella anteriorly.

Head circumference must be compared with that of mother to know any familial variations.
If the head circumference is more than expected, always measure the both parents' head circumference, for benign familial megalencephaly is common.
At birth , it is around 34 cm.HC increases 2cm/ month for the first 3 months, 1cm/month for next three months and 0.5 cm per month for next 6 months.
48-51 cm
38-40 cm
42-45 cm
33-35 cm