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 7.3 kg (16 lb 1 oz), while boys come in a little heavier at 7.93 kg (17 lb 5 oz). Length-wise, the average length for 6-month-old girls and boys is 65.73 cm (25.88 inches) and 67.62 cm (26.62 inches) 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 5.8 - 9.2 kg (12 lb 8 oz - 20 lb 3 oz) —and your health visitor will monitor that your baby is growing as expected at regular check-ups.