A 3-month-old boy weighs about 14.1 pounds (6.4 kg) on average, while a 3-month-old girl weighs about 12.9 pounds (5.8 kg). These are 50th percentile values from the WHO growth charts used by pediatricians, meaning half of all babies weigh more and half weigh less. A healthy 3-month-old can fall anywhere from roughly 10 to 18 pounds depending on sex, birth weight, and feeding patterns.
Average Weight by Sex
Boys and girls follow slightly different growth curves from birth. At 3 months, the typical ranges look like this:
- Boys: The 5th percentile is about 11.4 pounds (5.2 kg), the 50th is 14.1 pounds (6.4 kg), and the 95th is 17.2 pounds (7.8 kg).
- Girls: The 5th percentile is about 10.6 pounds (4.8 kg), the 50th is 12.9 pounds (5.8 kg), and the 95th is 15.8 pounds (7.2 kg).
Percentiles are not grades. A baby at the 15th percentile is not “failing” compared to one at the 80th. What matters most is that your baby follows a consistent curve over time rather than hitting one specific number. A baby who has tracked along the 20th percentile since birth is growing perfectly normally.
How Fast Babies Gain Weight at This Age
In the first few months of life, babies gain about 1 ounce (28 grams) per day. That pace begins to slow around 4 months, dropping to roughly 20 grams per day. So between months 2 and 3, you can expect your baby to put on somewhere around 1.5 to 2 pounds total.
This rapid early growth is why 3-month-olds have often nearly doubled their birth weight. A baby born at 7.5 pounds, for example, will commonly weigh 13 to 15 pounds by the 3-month mark. Babies who were smaller at birth may not have doubled yet, and that can be completely normal as long as their growth curve stays steady.
Breastfed vs. Formula-Fed Babies
Breastfed and formula-fed babies grow at slightly different rates, which can cause unnecessary worry if you’re comparing your baby to a friend’s. Healthy breastfed infants typically put on weight more slowly than formula-fed infants during the first year. The difference becomes more noticeable after about 3 months, when formula-fed babies tend to gain weight more quickly.
Both patterns are normal. The CDC recommends using the WHO growth charts for all children under 2, and those charts are based on breastfed infants as the standard. If your breastfed baby seems lighter than formula-fed peers, that alone is not a concern. Length growth is similar regardless of feeding method.
Growth Tracking for Premature Babies
If your baby was born early, the number on the scale at 3 calendar months may look lower than the averages above. That’s expected. Pediatricians use “corrected age” for premature babies, which means subtracting the weeks of prematurity from the baby’s actual age. A baby born 6 weeks early who is 3 months old by the calendar would be assessed as a 6-week-old for growth purposes.
Corrected age is used for growth tracking until age 2. Most premature babies experience catch-up growth that narrows the gap with full-term peers, typically by 12 to 18 months of age. Some continue catching up for several years. The goal is not always to reach the 50th percentile. For many preemies, especially those who were small for gestational age, the goal is simply to follow their own consistent curve.
How to Weigh Your Baby at Home
You don’t need a baby-specific scale. Stand on a standard digital bathroom scale and note your weight in kilograms. Then step on again while holding your baby naked. Subtract your weight from the combined weight, and you have your baby’s weight. A few tips to keep the reading accurate:
- Place the scale on a hard, flat surface. Carpet throws off the reading.
- Weigh baby naked and, if possible, before a feed.
- Be consistent. Weigh at roughly the same time of day each time.
- Don’t lean on anything or let your body touch a wall or counter.
- Wait for the number to stabilize if your baby is squirming.
Keep in mind that bathroom scales are less precise than the ones in your pediatrician’s office, so small differences of a few ounces between weigh-ins are not meaningful. Look at the trend over weeks rather than day-to-day fluctuations.
Signs of Slow Weight Gain
Most babies who weigh less than average are simply on the smaller end of normal. But certain patterns do signal a problem. A baby under 3 months old who gains less than 1 ounce per day may need a closer look from a pediatrician. Between 3 and 6 months, the threshold drops slightly: gaining less than about two-thirds of an ounce per day is considered slow.
A sudden change in trajectory is also worth noting. A baby who had been tracking steadily along a growth curve and then flattens out or drops to a significantly lower percentile may not be getting enough nutrition, or there could be an underlying issue affecting absorption or metabolism. One weigh-in that looks low is not cause for alarm, but a pattern over two or three visits is something your pediatrician will want to investigate.
Common reasons for slow gain at this age include difficulty latching, low milk supply, an undiagnosed tongue tie, or reflux that causes frequent spit-up. Most of these are treatable once identified, and babies typically bounce back to their expected curve within a few weeks of intervention.