The amount a three-month-old baby should weigh depends on the gender of the baby, explains BabyCenter. The average three-month-old boy weighs between 13 and 15.2 pounds. The average three-month-old girl weighs between 11.8 and 14 pounds.
These are the weight ranges of the 25th to 75th percentile for babies, says BabyCenter. Approximately one-quarter of babies weigh above, and approximately one-quarter of babies weigh below these ranges. No matter which percentile a baby is charted in, a lack of steady progress on the growth charts may alert the physician to look for and address any possibly related feeding, developmental or medical issues, states HealthyChildren.org.