Growth, 0-3 months Hey, chubby cheeks. At three months your little one is no longer, well, quite so little! According to the World Health Organization, 3-month-old girls weigh, on average, 5.8 kg (12 lb 7 oz), while boys come in at an average of 6.4 kg (14 lb 1 oz). Length-wise, the average at 3 months for girls and boys is 59.8 cm (23.5 inches) and 61.42 cm (24.2 inches) respectively. To give you a comparison, that’s about the same size as a small koala! Don’t get too caught up on these numbers (or how big a koala actually is). Your health visitor will monitor your baby’s growth at scheduled visits. You can also use these times as an opportunity to talk about any concerns you might have.