what is the size of the head in CM?
when my son is birth at 37w2days, his weight is only at 2.78kg as he is not a obesity baby means healthy baby(with diet control). my son head is at 32CM. Hence during labour it only took less than 5 minutes for my wife to give birth due to baby easy come out with NO major muscle tear or Birth complications. virginal heal faster also.
don't worry if the baby head is small, only worried if your baby heart 4 chamber is not normal, hand and leg bone is abnormal, spinal cord is not well develop, and etc.
As long as baby is healthy, small head is temporary, cos baby will grow at a very fast rate after they were born.