Most 4-month-old babies weigh between 12 and 17 pounds, with boys typically a bit heavier than girls. The average 4-month-old boy weighs about 14.3 pounds, while the average girl weighs around 13.2 pounds. But “average” is just the midpoint on a wide spectrum of healthy weights, and your baby’s individual growth pattern matters more than any single number.
Average Weight by Sex
The World Health Organization (WHO) growth standards, which the CDC recommends for all children from birth to age 2, place the median weight for a 4-month-old boy at roughly 14.3 pounds (6.5 kg) and for a girl at about 13.2 pounds (6.0 kg). Babies between the 3rd and 97th percentiles are generally considered within a normal range, which means a healthy 4-month-old boy could weigh anywhere from about 11.5 to 18 pounds, and a healthy girl from about 10.5 to 16.5 pounds.
These numbers reflect global data on healthy, breastfed infants growing under optimal conditions. Your pediatrician plots your baby’s weight on these charts at each visit, and the pattern over time is what they’re really watching.
The Doubling Birthweight Milestone
You may have heard that babies double their birth weight by 4 months. That’s a common benchmark, but the actual timeline is broader than many parents realize. Most babies double their birth weight somewhere between 4 and 6 months. A baby born at 7.5 pounds who weighs 13 pounds at their 4-month checkup hasn’t quite doubled yet, and that’s perfectly normal. By 6 months, the typical baby has reached that milestone.
During the first several months, babies gain roughly 1 to 1.25 pounds per month on average. That pace gradually slows in the second half of the first year as babies become more mobile and burn more energy.
How Feeding Type Affects Weight
Breastfed and formula-fed babies tend to follow noticeably different weight trajectories, and the gap becomes more apparent right around the 3- to 4-month mark. Breastfed babies typically put on weight more slowly than formula-fed babies after the first few months. Formula-fed infants gain weight more quickly starting around 3 months, and that difference persists even after solid foods are introduced later.
This doesn’t mean one feeding method produces healthier babies. The WHO growth charts are based on breastfed infants, so a formula-fed baby who looks like they’re climbing percentiles isn’t necessarily gaining too fast, and a breastfed baby tracking along a lower curve isn’t necessarily falling behind. What matters is that your baby follows a consistent trajectory rather than suddenly jumping or dropping across multiple percentile lines. Length growth, for what it’s worth, is similar between breastfed and formula-fed babies.
Why Percentile Trends Matter More Than One Number
A single weight measurement at 4 months tells you very little on its own. A baby consistently tracking along the 15th percentile is growing just as well as one tracking along the 85th. Both are following their own curve. Pediatricians look for a steady pattern across multiple visits, not one data point.
Some crossing of percentile lines is completely normal, especially in the first year as babies settle into their genetic growth pattern. A baby born large to smaller parents may gradually drift down, while a baby born small may climb. Where it gets more meaningful is when weight drops significantly over a period of a few months. If weight percentile drops first and height percentile follows a few months later, that pattern can suggest a baby isn’t getting enough calories. If weight and height fall off at the same time while the weight-for-length ratio stays normal, that’s a different picture and may point to a hormonal issue rather than a feeding problem.
Growth Spurts Around 4 Months
If your 4-month-old suddenly seems ravenous, fussy, or sleepier than usual, a growth spurt is a likely explanation. Babies go through several predictable growth spurts in the first year, with common ones occurring around 3 months and 6 months. While there isn’t a textbook spurt pinned to exactly 4 months, every baby’s timing varies, and a spurt in the 3-to-4-month window is common.
These episodes are short. In babies, growth spurts typically last up to about three days. During that window, your baby may want to eat more frequently and seem unusually clingy or restless. It passes quickly, and you may notice their clothes fitting a little tighter afterward.
What Influences Your Baby’s Weight
Genetics is the biggest factor. Larger parents tend to have larger babies, and smaller parents tend to have smaller ones, though this relationship isn’t always straightforward. Birth weight itself sets the starting point: a baby born at 9 pounds will likely weigh more at 4 months than one born at 6 pounds, even if both are growing at the same rate.
Prematurity also shifts the picture. Babies born early are typically plotted on growth charts using their corrected age (age from their due date, not their birth date) until around age 2. A baby born 6 weeks early and now 4 months old would be compared to the growth standards for a 2.5-month-old, which means a lower expected weight.
Illness, reflux, feeding difficulties, and milk supply issues can all slow weight gain temporarily. On the other hand, some babies are simply built stockier and consistently track above the 90th percentile without any overfeeding. As long as your baby is alert, meeting developmental milestones, producing enough wet diapers, and following a consistent growth curve, their specific number on the scale is far less important than the overall trend.