Ideal Weight and Length of 6 months old baby (girl & boy) The biggest parental concern centres on the baby’s health. At 6 months old, she might snooze around nine hours at night, sometimes even longer, with a few brief awakenings. During the day, she will still need around two or three naps. Girls (at 7 pounds, 1 ounce/3.2 kg) are a bit smaller than boys (at 7 pounds 4 ounces/3.3 kg) on arrival. At every medical visit, the doctor monitors the weight and height of the child and informs the parents if any actions have to be taken. Average or “normal” weight for a 6-month-old baby doesn’t tell the whole story though. Average baby weight and length chart by month In the United States, the average baby weighs about 7 pounds 3 ounces (3.3 kg) at birth. Average weight of a 6-month-old is 16.1 pounds for girls and 17.5 pounds for boys. In most cases, girls weight measurements for this age group (6 month old baby) will lie within the range between 5.73 and 8.93 kgs. Average length (height!) is 25.9 inches for girls and 26.6 inches for girls. When your baby is this age, she may sleep around 12 to 16 hours per day.