Hmm, untuk 3 bulan head circumference naik 1cm je dalam sebulan memang agak slow sikit, biasanya growth should be faster at this age. A few of us dengan babies around this age, ours semua naik lebih dari that monthly, tapi every baby lain lah. Better to check with your doctor or paediatrician next visit, they can review the growth chart and make sure baby's development on track, especially if you're worried. Don't stress yourself la, but good to confirm dengan the medical team just to be safe!