What does it mean if a baby's head circumference percentile drops from 90 to 75 at 5.5 months? does it indicate intelligence levels?

Head size: My first thought is technique and is a measuring error. That's the most likely. I would get it measured again and monitor it over the next few weeks to see if it is real or to watch the trend. Premature closure of the soft spot is a possibility which is fixable. Lower IQ would be unlikely. Talk to your doctor and let him know your concern and work together.