The average weight for a 5-month-old boy is about 16.5 pounds (7.5 kg), and for a girl it’s about 15.2 pounds (6.9 kg). These figures represent the 50th percentile on the WHO growth charts, meaning half of all babies weigh more and half weigh less. But “average” is just the midpoint of a wide healthy range, and where your baby falls on that range matters less than whether they’re growing consistently over time.
Healthy Weight Ranges at 5 Months
Most 5-month-old boys weigh between 13.2 and 20 pounds, while most girls fall between 12.1 and 18.4 pounds. That spread covers roughly the 5th to 95th percentiles. A baby at the 15th percentile is just as healthy as one at the 85th percentile, as long as they’ve been following a similar curve since birth.
The CDC recommends that U.S. pediatricians use the WHO Child Growth Standards for all children from birth to age 2. These charts are based on data from healthy breastfed infants across multiple countries and represent how children grow under optimal conditions. After age 2, providers switch to the CDC’s own growth reference charts.
What Growth Percentiles Actually Mean
A percentile describes where your baby sits compared to other babies of the same age and sex. If your baby is at the 30th percentile, that means 30% of babies weigh less and 70% weigh more. It does not mean your baby is underweight. Genetics, birth weight, and feeding method all influence where a baby naturally tracks on the chart.
What pediatricians watch for is the pattern over time, not a single number. A baby who has been cruising along the 20th percentile since birth is growing exactly as expected. The concern arises when a baby’s curve shifts significantly, either upward or downward, crossing through major percentile lines over a short period. Minor shifts are normal, especially during growth spurts, but a larger or sustained change can signal a feeding issue, illness, or other condition worth investigating.
Expected Weight Gain at This Age
Between 4 and 6 months, babies typically gain about 1 to 1.25 pounds per month. That’s noticeably slower than the rapid gains of the newborn period, when many babies pack on close to 2 pounds per month. This deceleration is completely normal. Growth doesn’t happen in a smooth, predictable line either. Babies grow in spurts, and a few days of extra hunger followed by a plateau is a common pattern.
Because growth is naturally uneven, the American Academy of Pediatrics recommends measuring weight at intervals of at least one month for babies under a year old. Weighing a baby too frequently, like daily or weekly at home, can create false alarms that reflect normal fluctuation rather than a real trend.
Breastfed vs. Formula-Fed Growth Patterns
Breastfed and formula-fed babies don’t grow at the same rate, and the difference becomes visible right around this age. Formula-fed infants typically gain weight more quickly after about 3 months, while healthy breastfed babies tend to put on weight more slowly throughout the first year. This difference persists even after solid foods are introduced.
This is one reason the WHO growth charts matter for babies under 2. Older charts were based heavily on formula-fed populations, which made breastfed babies look like they were falling behind when they were actually growing normally. If your breastfed baby drops from, say, the 50th percentile to the 35th between 3 and 6 months, that trajectory may be perfectly appropriate for how breastfed infants grow.
Premature Babies and Corrected Age
If your baby was born early, their weight at 5 months of calendar age won’t line up neatly with the charts. Pediatricians use “corrected age” instead, which accounts for the weeks your baby missed in the womb. The calculation is simple: subtract the number of weeks your baby was born early from their current age in weeks.
For example, a baby born at 32 weeks gestation was 8 weeks (about 2 months) early. At a calendar age of 5 months, their corrected age would be roughly 3 months, and their weight should be compared to the 3-month reference values, not the 5-month ones. Corrected age is used for growth tracking during the first 2 years, giving preemies a fairer comparison to their full-term peers.
What Influences Your Baby’s Weight
Several factors shape where your baby falls on the growth chart, and most of them are outside your control. Parental size is one of the strongest predictors. Tall, larger-framed parents tend to have bigger babies, and smaller parents tend to have smaller ones. Birth weight also sets a starting point: babies born on the larger side often stay in higher percentiles, though some regression toward the middle is common in the first few months.
Sex plays a role too. Boys are, on average, about a pound heavier than girls at 5 months, and that gap gradually widens. Feeding frequency, whether your baby has started showing interest in solid foods, recent illness, and even sleep patterns can cause short-term fluctuations. A baby recovering from a stomach bug might dip slightly and then catch up within a few weeks.
The key takeaway is that a single weight measurement is a snapshot. Your baby’s growth curve over several months tells a far more complete and meaningful story than any individual number on the scale.