Growth, 3-6 months Six months on and your little one will have well and truly outgrown their original onesie, and then some. According to the World Health Organization, the average weight for 6-month-old girls is 16lb 1oz (7.3kg), while boys come in a little heavier at 17lb 5oz (7.93kg). Length-wise, the average length for 6-month-old girls and boys is 25.88 inches (65.73cm) and 26.62 inches (67.62cm) respectively. No need to worry if your little one doesn’t match the middle mark. There’s a wide range of normal which most babies fit into—a healthy girl could weigh anywhere from 12lb 8oz - 20lb 3oz (5.8 - 9.2kg) — and your doctor will monitor your baby is growing as expected at regular check-ups.