Aiyooo mama, head circumference memang boleh vary dari baby to baby, tapi at 6 months 41cm is on the smaller side, better you check with your paediatrician to make sure everything's tracking okay, especially since your little one dah kena blood test. A few of us mums have had similar concerns and the doc just wanted to monitor closely, sometimes it's nothing, but it's good to get it confirmed so you can have peace of mind lah!