sounds as if babies head is a little smaller/larger compared to the body. From 37 weeks, baby is fully developed and will build up fatty tissue under the skin making them heavier. All babys put on most at the end of pregnancy, do not be alarmed, this is normal. Do not worry that your baby is small for now, see what doctors say after birth after they analise baby. All may be perfect so do not worry. Besides, labour will be easier for a smaller baby than a much larger baby, your girlfriend will be glad to hear :) everyone is different, and all babies are different. just make the most of spending time together before the birth x
They cant determine weight that accurately that far along in pregnancy I had an ultrasound at 37 weeks that said my daughter only weighted 5 lbs. 2 weeks later when she was born she was one ounces less then 7 lbs and in the 22nd percentile for weight.