Is my child's head size large for his age?

Q: My son is 15 months old and the diameter of his head is 51 cm. The paediatrician I go to tells me that this is not normal since it lies outside the normal range of 2 sigma of average sizes. She has asked me to get an MRI done and also consult a paediatric neurologist. We did notice the head being large and had an ultrasound done when he was 9 months old. It came out normal. Also, the size was 50 cms about 3 months back (which was also beyond the 97 percentile of the children). Is there anything alarming about this? He is an absolutely normal child and picked up all the activities at the right time (turning over/crawling/walking etc.). He now attempts to speak as well with words like ma-ma, pa-pa, etc. He seems like a very regular child. What should be my approach to this?

A:It is true that the head circumference of 51 cm at 15 months of age is larger than expected, beyond the 95th percentile.Normal patterns of head growth are as follows:Head circumference at birth for a full term normal baby: 35 cms.Rate of increase of head circumference with age:0 to 3 months: 2 cm / month4 to 6 months: 1 cm / month6 to 12 months: 0.5 cm / month12 - 24 months: 2 cm / year.However, when there is increased head circumference, the following factors are used to decide if investigations such as MRI or CT (neuroimaging) are needed:1. Physical examination: If the child has abnormal features such as large bulging soft spot on the skull (bulging fontanelle or split sutures), abnormal muscle tone or weakness, visual problems, etc.. 2. Developmental milestones: If the child has delayed milestones, such as not walking at 12 to 15 months, not crawling at 8 to 9 months, not saying mama, dada at 10 to 12 months etc.3. Rate of head growth: While plotting the head size on growth charts, if the trend is a sudden increase compared to the child's previous rate of head growth. If the rate of growth is steady according to the figures quoted above, and remains stable paralleling a normal growth curve but above it (without sudden jumps), neuroimaging may not be needed. In summary, if the physical exam of the nervous system is normal, milestones are appropriate, and the head circumference though above the 95th percentile is not increasing rapidly when plotted on the growth charts, only close clinical follow-up is needed by the paediatrician.An additional reassuring factor may be finding out if parents had big heads as children. This is called benign familial megalencephaly and accounts for >95% of large heads. Also, if neuroimaging has to be done, it should be preferably MRI as ultrasound is not very useful.