Most four-month-old babies weigh between 12 and 16 pounds, though healthy weights vary based on sex, birth weight, and feeding method. A practical benchmark: by four months, most full-term babies have doubled their birth weight. So if your baby was born at 7 pounds, you’d expect them to be around 14 pounds now.
Average Weight by Sex
Boys and girls follow slightly different growth curves at this age. According to the WHO growth charts used by most pediatricians, the 50th percentile weight for a four-month-old boy is about 15.2 pounds (6.9 kg), while for a girl it’s about 13.9 pounds (6.3 kg). The normal range is wide, though. A boy anywhere from roughly 12.6 to 17.4 pounds falls between the 10th and 90th percentiles. For girls, that range is about 11.4 to 16.1 pounds.
These numbers are reference points, not targets. What matters more than any single measurement is whether your baby is following a consistent curve over time. A baby who has tracked along the 20th percentile since birth is growing perfectly well, even though they weigh less than most babies their age.
How Fast Babies Gain at This Age
Between four and six months, babies typically gain about 1 to 1.25 pounds per month. That’s noticeably slower than the rapid gains of the first three months, when many babies put on close to 2 pounds monthly. This is completely normal. Growth naturally decelerates as babies get older, and the slowdown becomes even more pronounced after six months when solid foods enter the picture.
The doubling-of-birth-weight milestone is one of the easiest ways to check that your baby is on track. If your baby was born at 6.5 pounds and now weighs 12 pounds at four months, that’s a healthy trajectory even though it’s below the 50th percentile on the chart.
Breastfed vs. Formula-Fed Babies
Breastfed and formula-fed babies grow at different rates, and the gap becomes visible right around this age. Breastfed babies typically put on weight more slowly than formula-fed babies starting after about three months. This isn’t a sign that breast milk is insufficient. It reflects normal biological differences in how the two types of feeding regulate calorie intake. Formula-fed babies tend to gain weight more quickly from this point forward, and the differences in weight patterns persist even after both groups start eating solid foods.
The WHO growth charts, which are based primarily on breastfed infants, are the standard in the U.S. for children under two. If your pediatrician uses the older CDC charts (based on a mix of breastfed and formula-fed babies), a breastfed baby might appear to be “falling behind” when they’re actually growing exactly as expected. It’s worth asking which chart your doctor uses if you’re told your breastfed baby’s weight is a concern.
What If Your Baby Was Born Early
Premature babies are evaluated using their “adjusted age” rather than their calendar age. If your baby was born two months early and is now four months old, their growth is compared against what’s expected for a two-month-old. Pediatricians typically use adjusted age for growth assessments until a preemie is about two years old.
For very premature infants (born before 37 weeks), doctors use specialized growth charts like the Fenton Preterm Growth Charts, which track weight, length, and head circumference by gestational age. These charts bridge the gap between preterm expectations and the standard WHO growth standards, giving a more accurate picture of whether a premature baby is growing well. If your baby was born early and you’re comparing their weight to the numbers in this article, the comparison won’t be meaningful without adjusting for prematurity first.
When Weight Actually Signals a Problem
The rate of change in your baby’s weight matters more than any single number on the scale. A baby who has been tracking along the 15th percentile for months is in a very different situation from a baby who was at the 60th percentile and has dropped to the 15th. That kind of downward crossing, especially a drop of two or more major percentile lines, is what prompts pediatricians to investigate further.
Clinically, weight that falls below the 3rd percentile, or weight that’s more than 20% below what’s expected for the baby’s length, can indicate a condition called failure to thrive. But the label is less about a specific number and more about a pattern: growth that has slowed or stopped after previously following an established curve. Common causes range from feeding difficulties and food sensitivities to digestive issues, and most are very treatable once identified.
Signs you might notice at home include a baby who seems unusually disinterested in feeding, isn’t producing at least six wet diapers a day, or seems lethargic compared to their usual energy level. If your baby is gaining weight steadily, meeting developmental milestones, and feeding well, a number that looks low on the chart is rarely cause for concern on its own.
Making Sense of Percentiles
Growth chart percentiles tell you how your baby compares to other babies of the same age and sex. A baby at the 30th percentile weighs more than 30% of babies and less than 70%. That’s it. Being at the 30th percentile doesn’t mean your baby is underweight, and being at the 90th doesn’t mean they’re overweight. The charts describe the full range of normal.
Your baby’s individual curve is what your pediatrician watches most closely. Babies tend to find their growth channel in the first few months and then follow it with reasonable consistency. Some babies are genetically smaller and will track the lower percentiles their entire childhood. Others are naturally larger. Both patterns are healthy as long as the trajectory stays relatively stable. The four-month checkup, which most pediatric offices schedule, is a good opportunity to look at your baby’s curve and ask questions about how their growth is trending.