Both of my boys have had large head circumference and by 3 months were literally off the charts.

Both were referred to a paed (initially for other reasons) who tracked their growth (DS1 until 12 months, DS2 is only 4 months old and we have to go back in 3 months time) and both boys had u/sounds and xrays to rule out any nasties (extra fluid around the brain mainly).

If you are concerned, speak to your MCHN or GP about getting some u/sounds done.

In our case, both boys have been fine, they just have huge heads, as does their father and me too apparently In our case their heads were pretty well in proportion with their bodies and all their other measurements (weight, length etc ) were at least 90th percentile.