Understanding baby size
Babies come in all shapes and sizes. Weight can vary drastically. The average weight for full-term babies is 7 pounds, 5 ounces. However, a percentage of healthy, full-term babies are born under or over that average weight.
As your baby grows, their rate of weight gain will be an important indicator of overall health and development. Your baby’s pediatrician will monitor weight, length, and head size at each well-child appointment to determine if your baby is progressing as they should.
Read on to learn more about the average weights for different ages.
The following weights for
|Age||50th percentile weight for male babies||50th percentile weight for female babies|
|Birth||7.8 lbs. (3.5 kg)||7.5 lbs. (3.4 kg)|
|0.5 months||8.8 lbs. (4.0 kg)||8.4 lbs. (3.8 kg)|
|1.5 months||10.8 lbs. (4.9 kg)||9.9 lbs. (4.5 kg)|
|2.5 months||12.6 lbs. (5.7 kg)||11.5 lbs. (5.2 kg)|
|3.5 months||14.1 lbs. (6.4 kg)||13 lbs. (5.9 kg)|
|4.5 months||15.4 lbs. (7.0 kg)||14.1 lbs. (6.4 kg)|
|5.5 months||16.8 lbs. (7.6 kg)||15.4 lbs. (7.0 kg)|
|6.5 months||18 lbs. (8.2 kg)||16.5 lbs. (7.5 kg)|
|7.5 months||19 lbs. (8.6 kg)||17.4 lbs. (7.9 kg)|
|8.5 months||20.1 lbs. (9.1 kg)||18.3 lbs. (8.3 kg)|
|9.5 months||20.9 lbs. (9.5 kg)||19.2 lbs. (8.7 kg)|
|10.5 months||21.6 lbs. (9.8 kg)||19.8 lbs. (9.0 kg)|
|11.5 months||22.5 lbs. (10.2 kg)||20.7 lbs. (9.4 kg)|
|12.5 months||23.1 lbs. (10.5 kg)||21.4 lbs. (9.7 kg)|
When tracking your baby’s growth, their doctor will likely use a graph from the CDC, if you live in the United States. If you live outside of the U.S., they’ll use another national organization that shows the weight, length, and head circumference in terms of percentile.
Your baby may be in the 25th percentile for weight, for example. That means that 75 percent of babies at the same age weigh more than your baby and 25 percent weigh less.
Being in a low or high percentile doesn’t mean that your baby will be underweight or overweight for their entire life. It also doesn’t mean that there’s anything wrong with your baby.
Instead, doctors are interested in tracking whether your baby drops into a lower percentile over time. This may be a sign of failure to thrive. If they suddenly jump into a new percentile, this shows that they may have had a growth spurt.
Research shows newborn, healthy babies typically lose some of their birth weight in the days following delivery. This is largely because they’re born with extra fluid. They eliminate this fluid quickly.
Babies begin gaining weight shortly afterward. The ounces they lose at birth are usually regained
During their first month of life, babies gain around 5 to 7 ounces a week, according to the Mayo Clinic.
Right before or during a growth spurt, your baby may be fussier than usual. They may also eat more or cluster feed. Cluster feeding is when they breastfeed more frequently for set periods of time (clusters). They may also sleep more or less than usual.
After the growth spurt, you may notice that their clothing no longer fits. They’re ready to move into the next size up.
Babies also experience periods of time when their weight gain may slow down.
Over their first few months, boys tend to gain more weight than girls do. But most babies double their birth weight by 5 months of age.
Babies who are born prematurely often, though not always, weigh less than full-term babies. A baby is considered full-term if they’re born at or after 39 weeks of gestation.
Every week makes a difference. A baby born at 24 or 25 weeks will weigh less than a baby born at 28 or 29 weeks.
If your baby is premature, they may have a low birth weight or very low birth weight:
- Babies born at a low birth weight weigh between 3 pounds, 5 ounces (1.5 kilograms) to 5 pounds, 8 ounces (2.5 kilograms) at birth.
- Babies born at a very low birth weight weigh less than 3 pounds, 5 ounces at birth (1.5 kilograms) at birth.
Premature babies require more medical attention and support when they’re born. They often stay in the neonatal intensive care unit (NICU) until they’re healthy enough to go home. This often occurs near their original due date.
The ability to gain weight steadily will be necessary before your baby can go home. Often, though not always, babies are kept in the NICU until they weigh at or near 5 pounds.
Just like all babies, preemies lose some weight after birth and then begin to gain the weight back. While your baby is in the NICU, you’ll likely be able to supply them with pumped breast milk.
Babies don’t develop the sucking reflex until 32 weeks old, so babies born very early are given milk through a tube into their stomach at first. Your baby can also drink formula this way.
Weight gain is an important measure of health for premature infants. If there are no underlying health issues that make it difficult for your baby to grow, they’ll gain weight steadily.
Based on their level of prematurity, for the first few weeks, the amount of weight gain may be similar to the amount of weight they’d be gaining were they still in utero.
Premature babies grow and gain weight at a faster rate than full-term babies do. During their first year, premature babies are measured for weight based on the age they would’ve been born at term rather than by their actual birth date.
For example, if your baby is born at 35 weeks, when they’re 5 weeks old, their doctor will refer to the newborn weight percentiles instead of those for a baby that’s 5 weeks old.
Many premature babies catch up to full-term babies in terms of weight by their first birthday. Some may not catch up until they’re 18 to 24 months old.
Your newborn’s weight is determined by many factors. These include:
- Genetics. For example, the size of each birth parent.
- Length of pregnancy. Babies born before their due date are often smaller. Babies born past their due date may be larger than average.
- Nutrition during pregnancy. A healthy diet while pregnant helps your baby grow in your womb and beyond.
- Lifestyle habits during pregnancy. Smoking, drinking alcohol, or taking recreational drugs can affect your baby’s birth weight.
- Your baby’s sex. It’s a small difference at birth, but boys tend to be larger and girls smaller.
- Health conditions of the birth mother during pregnancy. Conditions such as diabetes, gestational diabetes, heart disease, high blood pressure, and obesity may affect your baby’s weight.
- Number of babies in the womb at one time. Singleton, twins, triplets, or more can affect your baby’s weight, depending on how much space they have to share.
- Birth order. Firstborns can be smaller than their siblings.
- Your baby’s health. This includes medical issues such as birth defects and exposure to infection during gestation.
Your baby’s weight is one of many important measures your doctor can use to help determine if your baby is developing as expected or if there may be an underlying concern.
Health concerns for underweight babies
Babies may have trouble gaining weight for a variety of reasons. These include:
- difficulty suckling
- not getting enough daily feedings or calories
- vomiting or gagging on milk
- exposure to a prenatal infection
- birth defects, such as cystic fibrosis
- medical conditions, such as gastroesophageal reflux or congenital heart disease
When a baby doesn’t gain weight normally, it can signal problems such as undernutrition or an underlying health issue.
Not being able to gain weight is a concern because it can affect your baby’s ability to hit developmental milestones. It may also have an adverse impact on their immune system.
Health concerns for overweight babies
If you have diabetes or gestational diabetes, you may have a larger baby. Above average-weight babies may require extra medical attention to make sure their blood sugar levels are kept in a normal range.
Your baby may also weigh more than average if you gain more than the recommended amount of weight during pregnancy. This is one of the reasons why it’s important to maintain a healthy diet while you’re pregnant.
In the United States, pregnant women are typically recommended to gain between 25 to 30 pounds during pregnancy. Your doctor may recommend you gain more or less than that depending on your weight and health pre-pregnancy.
Always follow your doctor’s recommendations.
Increased weight gain in babies in the first 6 to 12 months of life isn’t usually a concern.
Breastfed babies, in particular, often gain weight more rapidly in the first 6 months, then slow down after that. Occasionally, babies who weigh more may crawl and walk later than other babies do.
It’s important to help your baby stay within a healthy weight as they grow and start solid foods. Doing so may help them maintain a normal weight later on. Talk to their doctor if you’re concerned about their weight.
If you’re concerned that your baby is underweight or overweight, talk to their doctor. They can show you your baby’s rate of growth and, if necessary, work with you on a nutritional therapy plan. This type of plan can help you determine the number of feedings you should supply per day.
If your baby is having a difficult time gaining weight and your supply of breast milk is low, your doctor may recommend to supplement breast milk with formula. It’s usually recommended to wait until your baby is 6 months old to start solids, like rice cereal or purees.
If your baby is having trouble suckling, consider working with a lactation consultant. They can help you find comfortable positions to hold your baby and provide suggestions and support to make breastfeeding successful for you and your baby.
There are also suckling exercises you can try that may help them take your breast or a bottle more readily. Examples include massaging your baby’s chin or tapping on their lips.
One way to determine if your baby is taking in enough nutrition is to monitor the number of bowel movements and wet diapers they produce daily:
- Newborns may have as few as one or two wet diapers daily and produce stool that’s very black in color.
- By 4 to 5 days old, babies should have six to eight wet diapers and several soft, yellow stools every 24 hours.
- Infants and babies who are 1 to 2 months old may produce between four and six wet diapers daily and have three or more bowel movements each day.
The number of daily bowel movements tends to decline as babies get older. If your baby’s urine or stool output is low, they may not be getting enough nutrition. They may benefit from additional feedings.
It’s also important to monitor your baby’s reflux. Though rare, if they’re spitting up as much as they’re taking in, they probably aren’t getting enough nutrition.
Try smaller, more frequent feedings, with lots of added time for burping. This may help your baby keep down breast milk or formula.
Your baby’s weight is one of many important indicators that help you and your baby’s doctor track your baby’s development. Gaining weight too slowly or too quickly can have long-term health consequences if not addressed.
However, a baby’s weight at birth doesn’t indicate what their weight will be as adults. Babies born prematurely or at a low birth weight can quickly catch up to their peers. Older babies and toddlers who are overweight can get help to reach and stay within a healthy weight range.