At 4 months old, the average baby boy weighs about 15 pounds (6.8 kg) and the average baby girl weighs about 14 pounds (6.4 kg). But healthy weights at this age span a wide range, roughly 12 to 18 pounds, depending on birth weight, feeding method, and genetics. What matters most isn’t hitting one specific number. It’s whether your baby is following a consistent growth curve over time.
Average Weight by Sex
The World Health Organization growth standards, which the CDC recommends for all children under age 2, place the 50th percentile for 4-month-olds at approximately 15.4 pounds (7.0 kg) for boys and 14.1 pounds (6.4 kg) for girls. The 50th percentile simply means half of healthy babies weigh more and half weigh less. A baby tracking along the 15th percentile is just as healthy as one at the 85th, as long as they’re growing steadily along their own curve.
Here’s a rough breakdown of what different percentiles look like at 4 months:
- Boys, 5th percentile: about 12.7 lbs (5.8 kg)
- Boys, 50th percentile: about 15.4 lbs (7.0 kg)
- Boys, 95th percentile: about 18.3 lbs (8.3 kg)
- Girls, 5th percentile: about 11.7 lbs (5.3 kg)
- Girls, 50th percentile: about 14.1 lbs (6.4 kg)
- Girls, 95th percentile: about 17.0 lbs (7.7 kg)
Your baby’s birth weight sets the starting point. A baby born at 6 pounds will naturally weigh less at 4 months than one born at 9 pounds, and both can be perfectly on track.
How Fast Babies Gain Weight
In the first three months of life, babies gain about 1 ounce (28 grams) per day. That’s roughly 2 pounds per month, which is a surprisingly fast rate relative to their size. Around the 4-month mark, that pace slows to about 20 grams per day, or just under 1.5 pounds per month.
A common milestone parents hear about is doubling birth weight. Most babies reach that point by 5 to 6 months, not at 4 months. So if your baby was born at 7.5 pounds and weighs 13 pounds at their 4-month checkup, they’re right on schedule even though they haven’t doubled yet.
Breastfed vs. Formula-Fed Growth Patterns
Breastfed and formula-fed babies don’t gain weight at the same rate, and this is completely normal. For the first three months or so, growth looks similar. After that, formula-fed infants typically gain weight more quickly. Breastfed babies put on weight more slowly through the rest of the first year, and these differences persist even after solid foods are introduced.
This means a breastfed baby who dips from the 50th percentile to the 35th between 3 and 6 months isn’t necessarily falling behind. It may simply reflect the natural growth pattern of breastfed infants. This is one reason the CDC recommends using the WHO growth standards for children under 2: those charts were built from data on breastfed babies and better reflect normal variation. If your pediatrician is using older charts based primarily on formula-fed infants, a healthy breastfed baby can look like they’re lagging when they’re not.
Length growth, on the other hand, is similar between both feeding methods.
What Pediatricians Actually Look For
Your baby’s doctor isn’t focused on one weigh-in. They’re looking at the trend across multiple visits. A baby who has been tracking along the 25th percentile since birth and stays there is growing beautifully. A baby who drops from the 60th to the 15th percentile over two visits is more concerning, even if their current weight is technically “normal.”
Pediatricians watch for a pattern called crossing percentile lines, where a baby’s weight falls across two or more major percentile curves on the growth chart. A single dip can happen during an illness or a growth spurt in length (babies sometimes grow taller before filling out). But a sustained downward trend over weeks or months may signal that a baby isn’t getting enough nutrition.
Between checkups, some practical signs that your 4-month-old is eating enough include several wet diapers per day (at least 4 to 6 heavy ones), regular bowel movements, and a baby who seems satisfied after feedings and is alert and active during awake periods.
How to Weigh Your Baby at Home
If you want to track weight between pediatric visits, a digital baby scale gives you the most accurate reading. These range widely in price, but even a basic model will work for monitoring trends.
If you don’t have a baby scale, you can use a regular bathroom scale. Weigh yourself first and note the number. Then weigh yourself holding your naked baby. Subtract your weight to get the baby’s weight. This method is less precise, especially for detecting small changes of a few ounces, but it gives a reasonable ballpark.
One tip for checking your scale’s accuracy: weigh a known object, like a 1-kilogram bag of sugar. If the reading is close to 1 kg, your scale is reliable enough for home tracking. Keep in mind that small fluctuations from feeding to feeding or morning to evening are normal. Try to weigh at roughly the same time of day, before a feeding, for the most consistent comparison.
When Weight Gain Might Be a Concern
A 4-month-old who is gaining very little weight, losing weight, or whose growth curve has flattened out may need further evaluation. Some common and fixable reasons for slow weight gain at this age include difficulty with latch or milk transfer during breastfeeding, not feeding frequently enough, or reflux that causes a baby to take in less than they need.
Premature babies follow their own timeline. If your baby was born early, your pediatrician will likely use an adjusted age (based on your due date, not your delivery date) to plot growth. A baby born 6 weeks early and now 4 months old would be compared to the growth chart at roughly 2.5 months of adjusted age.
On the other end of the spectrum, rapid weight gain in infancy is rarely a concern on its own. Babies don’t need to be put on diets. If your baby is above the 95th percentile but following a steady curve, that’s their growth pattern. Overfeeding is difficult with breastfeeding and uncommon even with formula when parents follow hunger cues rather than rigid schedules.