Every appointment I went to at the MCHN my DD head size was commented on, and like you she was vague really about why is a problem (hydrocephalus seems to be the main thing they are concerned about which is excess fluid on the brain) at 6 months check she said we should mention it to GP next time we visited (we are not frequent visitors to GP so I pressed as to whether should make an appointment and she said yes), saw GP who said yes DD head is large and said to watch for other symptoms (sickly, bulging eyes, bulging fontanelle) and if at next measurement (2 months down line) was off the chart would refer me to a paeditrician. Next measurement she was off chart, different MCHN said 90% of big heads are due to a family reason, but should go back to GP as that is what had been agreed. Saw Paed this week, she basically just checked on DD's development for her age and looked at the curve of her head growth. She is now following a curve but her curve is above 97th percentile, she measured my head (57.4cm) and DH (60cm) and said that DD looks fine she just has a big head due to genetics . If there is a ever a spike in her measurements then we should go back, but if she is following the same curve then all is good. Her head growth spurt seemed to be between 3 and 7 months.(My DD is also 75th for weight and height, hc started at 35 and is now 47).

So I would measure both parents heads, as that made me feel more comfortable about it, and just look out for any of the symptoms (e.g. sicklyness, bulging eyes, fontanelle that is bulging all the time (not the pulsing that is normal)) until the next measurement (I also used to measure it myself just to reassure myself as found I did fixate a bit on the size of her head). As Curly said and as the MCHN and Paed told me most cases large heads are due to family genetics but they just have to be cautious that isn't due to fluid.