The mean or average birth weight in the United States is approximately 7.5 pounds (3,400 grams). However, average does not necessarily mean normal. A birth weight between 5.5 pounds (2,500 grams) and up to 10 pounds (4,500 grams) is considered to fall in a normal range for a full-term newborn. Newborns that are on the smaller side might be referred to as low birth weight or small for gestational age. Babies that are larger are considered to be large for gestational age. Learn more in this article about the ranges of healthy birth weight and when to be concerned.
Baby Birth Weight Statistics
According to a detailed report from the National Center for Heath Statistics published by the Centers for Disease Control and Prevention (CDC), there were 3,664,292 births registered in the United States in 2021. Although babies come in many sizes (from just under 1 pound to more than 16 pounds) data from 2021 revealed that 297,604 babies were born at low birth weight. This makes up about 8% of all babies. About 1% of babies were born weighing less than 1500 grams. These babies are considered to be born at very low birth weight and often face medical complications. In some cases, what is termed low birth weight is actually appropriate. For example, if a baby born is premature (less than 37 weeks gestation) they would “normally” weigh less than 5 pounds, 8 ounces (2,500 grams). Keep in mind that birth weight numbers are derived from special scales—they are more accurate (and regulated) than a bathroom scale you’d use at home. If you’re concerned about your baby’s birth weight, or their weight as they continue to grow, discuss these concerns with your child’s pediatrician.
Baby Birth Weight Terminology
There are different terms that are used to describe birth weight. When babies are born preterm or postdate (overdue), the terms can become a little confusing. Rather than using absolute weight, the terms that are used to describe gestational age more accurately reflect a baby’s size. Depending on a baby’s weight at birth and their gestational age, a special growth chart is used to classify infants into one of the following categories.
Birth Weight
Birth weight has five main categories: normal, high low, very low, and extremely low. Normal birth weight is between 5 pounds, 8 ounces (2,500 grams) and 8 pounds, 13 ounces. High birth weight (HBW) is more than 8 pounds, 13 ounces (4,000 grams). Low birth weight (LBW) babies weigh less than 5 pounds, 8 ounces (2,500 grams). Very low birth weight (VLBW) babies weigh less than 3.4 pounds (1,500 grams), and extremely low birth weight (ELBW). babies weigh less than 2 pounds (1,000 grams)
Gestational Age
Babies born at a younger gestational age will generally be smaller and babies who are born at an older gestational age will generally be larger. But if the size does not match with gestational age, they may be considered small or large for gestational age. Babies whose birth weight ranges from the 10th percentile to the 90th percentile relative to other babies born at that gestational age are considered appropriate for gestational age (AGA). Those whose birth weight is less than the 10th percentile for a child born at that gestational age are considered small for gestational age. When birth weight is greater than the 90th percentile based on gestational age, this is considered large for gestational age (LGA). The term fetal macrosomia is also used for LGA babies. The term intrauterine growth restriction (IUGR) is sometimes used to describe a baby with a birth weight lower than expected for gestational age. However, it is most often used to describe a fetus that is growing less than expected during pregnancy.
Why Are There Different Classifications?
Birth weight and gestational age classifications are useful because they often correspond with clinical care and treatment. Many of the terms listed above can be used together. For example, a premature baby could be born with low birth weight (or even extremely low birth weight) but still be at an appropriate weight for their gestational age. On the other hand, a full-term baby born at 5.5 pounds (2,500 grams) would likely be classified as being both SGA and IUGR.
Factors Affecting Baby Weight at Birth
There are many different factors used to determine a baby’s birth weight, including age, genetics, and certain lifestyle factors of the mother.
Age
Teens tend to have smaller babies, as do those over age 35. However, research has also shown a connection between advanced age and high birth weight as well.
Genetics
Genetics also play a role in birth weight. The genetic characteristics of both parents are important. One difference, however, is that the mother’s weight at her own birth has a greater impact than the father’s birth weight.
Smoking
Smokers tend to have smaller babies, as physiological changes related to smoking reduce the nutrients supplied to the baby. Exposure to secondhand smoke is also correlated with low birth weight and other complications such as IUGR.
Nutrition
Nutrition during pregnancy can also affect an infant’s birth weight. Weight gain in pregnancy is influenced by different factors, including her socioeconomic conditions, pregnancy and non-pregnancy related health conditions, and genetics.
Prenatal Care
A lack of early and regular prenatal care has been associated with lower birth weight babies. A lack of prenatal care might result from poor access to health care (for example, options are limited by geographic location), mental health concerns, or socioeconomic conditions.
Overall Parental Health
The health of an infant’s biological parents can affect a newborn’s birth weight. People who are heavier when they become pregnant may have larger babies. Having a history of high blood pressure before pregnancy has been associated with smaller babies. Having a history of diabetes (preexisting diabetes) is associated with larger-than-normal babies. Pregnancy-induced hypertension or PIN (high blood pressure during pregnancy) and gestational diabetes (diabetes related to pregnancy) also affect birth weight. PIN is associated with smaller babies and gestational diabetes is linked to large-for-gestational-age babies. Certain hereditary uterine conditions (such as a bicornuate uterus), as well as acquired conditions (fibroids), can result in lower birth weight. Alcohol and drug use can also affect the birth weight of a baby, typically leading to smaller birth weights.
Other Factors
While many factors can be modified, there are some factors that cannot be changed. For example, male infants tend to weigh slightly more at birth than female infants. First babies tend to weigh less than subsequent babies and twins and other multiples are usually smaller than singletons.
Monitoring Newborn Weight Gain
If your baby is full-term, of normal birth weight, and has no medical conditions, it’s not usually necessary to weigh your baby as long as they are eating well, have wet diapers, and are developing normally. Your pediatrician will check your newborn’s weight at each well-child visit and let you know if there is any cause for concern. If your infant is low birth weight, was born prematurely, or your pediatrician has any other concerns, you will likely be asked to make more frequent visits to the clinic for weight checks. Average weight gain can vary for children who are born with low birth weight or are large for gestational age. For example, premature babies often undergo catch-up growth. If your baby was born early, your pediatrician will explain growth expectations. Otherwise, your baby’s weight gain by age can be monitored using the following guidelines.
Initial Weight Loss
Babies usually lose weight at first. This weight loss is roughly 5% of body weight in babies who are bottle-feeding and 7 to 10% in babies who are breastfeeding. Babies usually regain their birth weight by 10 to 14 days of age. Pediatricians will often give the green light to stop waking your baby up every three to four hours during the night for a feed at this point.
The First 3 Months
In the first 3 months of life, babies gain an average of 1.5 to 2 pounds per month and grow an average of 2 centimeters (around 1 inch) per month. Your pediatrician will talk about normal growth rates for young children. Your doctor can also show you where your child is on a growth chart—a graph that compares your baby’s height and weight to other babies of the same age.
4 to 6 Months
Between the age of 4 months and 6 months, babies put on weight less rapidly at around 1 to 1.25 pounds per month and are growing 1/2 to 1 inch each month. By around the 5-month mark, a baby’s birth weight is usually doubled.
6 Months to 1 Year
Weight gain begins to slow down between 6 months and 9 months of age, with growth in length being roughly 3/8 of an inch (1 centimeter) per month from 6 months to 12 months. Birth weight is usually tripled by around 1 year of age.
1 to 2 Years
On average, your baby’s weight will roughly quadruple by the time they are about 2 and a half years old. At this age, there are calculations you can use to estimate your child’s adult height.
A Word From Verywell
The birth weight of babies can vary and will be affected by many factors. A baby’s birth weight does not necessarily predict a child’s adult size. Some very-low-birth-weight babies grow up to be quite tall or large, while large-for-gestational-age babies might be small adults. Whatever your baby’s birth weight is, your pediatrician will help you understand the growth expectations for your baby. They can use graphs to show you where your child falls in terms of their growth, and how their growth might affect their health.
