If there is anything that experts can agree on it is that every baby grows and develops at their own rate. Doctors keep a close eye on your baby's progress from birth to ensure that no troubling patterns arise. Head circumference is just one of the many routine tests and measurements that are used to help your doctor or pediatrician gauge your child's health and there are a number of factors that can affect that reading.
First and foremost, your doctor is looking less at the actual numbers and more at the pattern on the growth chart. If your baby was born at the 10th percentile and continued along at that curve, remaining a little on the small side, this would be no cause for concern. However, if your child was born at the 50th percentile and at their 4 month check-up dropped down to the 5th in one of the criteria (such as head growth), this would be cause for concern. Most likely if they drop in one area, they drop in all of them. And often this can be repaired by looking at their diet. Perhaps it is time to start supplementing with formula or begin solids to ensure that your baby is continuing along the pattern they earlier established. It is not uncommon for a baby to slow down in their growth curve, especially once they start crawling or walking as they begin burning off more calories and sometimes are less interested in eating and more interested in practising their new skills. There are many variables involved, however if your child has dropped in their percentile, this is cause for an evaluation not necessarily concern.
One of the largest factors that can effect a reading of head circumference is the margin of error. As with measuring uterine growth in pregnancy, measuring a baby's head growth can be inaccurate. The measuring tape should be around the forehead in the front and the largest part of the back, if it is off by even a little bit, it can have a huge impact on the reading. So don't panic! Simply ask if the measurement can be taken again, or consult your pediatrician to see if maybe one of the previous measurements could have been off thus causing what appears to be a drop in their development.
There are a number of factors that can affect normal head growth including inadequate nutrients in pregnancy, drugs or smoking in pregnancy, etc. No matter what your situation, sit down with your doctor to discuss the possibilities as to what you can do to help remedy the situation (if indeed there is a situation to worry about). Most importantly, continue to have regular check-ups with your doctor to monitor your child's progress as they grow.