Baby Birth Weight Statistics

The definition of average, normal, small, and large babies

Baby birth weights in the US

Verywell / Nusha Ashjaee 

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.

The 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 3,853,472 births registered in the United States in 2017.

In the U.S., state laws require that birth certificates are completed for all births. Federal law mandates the collection and publication of all births and other vital statistics data, which is then compiled by The National Vital Statistics System (NVSS) to provide statistical information from birth certificates.

Here is some information about the birth population gleaned from the data.

Averages

Although babies come in many sizes (from just under 1 pound to more than 16 pounds) data from 2017 revealed that:

  • The average weight at birth was between 6 pounds, 9 ounces (3,000 grams), and 7 pounds, 11 ounces (3,500 grams).
  • 8.28% of babies were considered to be low birth weight (defined as less than 5.5 pounds or about 2,500 grams).
  • 1.4% of babies were very low birth weight (less than 3.3 pounds or 1,500 grams).
  • Roughly 9% of babies were large for gestational age at birth.
  • The average length of a full-term infant was 20 inches.
  • The normal range for full-term infants was 18 inches to 22 inches.

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.

Interesting Trends

Research has shown that kids and adolescents are getting bigger—a trend that is termed the childhood obesity epidemic. Based on findings in older children and teems, it would be natural to assume that newborn babies are also getting bigger.

However, statistics show that babies are actually getting smaller. It's unclear as to why, since research has not shown a direct link between lower mean birthweight and an increase in premature babies, nor is there a direct correlation to other independent factors such as more Cesarean births.

While the exact cause of the decline in birth weight is unknown, it could be attributed to trends in maternal diet, physical activity, socioeconomic factors, environmental exposures, or even other, unrecorded medical conditions.

Recent Trends in Average Birth Weight

  • 1990: 7 lbs., 9.4 oz (3,441 g)
  • 1995: 7 lbs., 9.17 oz (3,435 g)
  • 2000: 7 lbs., 8.95 oz (3,429 g)
  • 2005: 7 lbs., 7.54 oz (3,389 g)

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:

  • Extremely low birth weight (ELBW). Birth weight less than 2 pounds (1,000 grams)
  • Very low birth weight (VLBW). Birth weight less than 3.4 pounds (1,500 grams)
  • Low birth weight (LBW). Birth weight less than 5 pounds, 8 ounces (2,500 grams)
  • Normal birth weight. Between 5 pounds, 8 ounces (2,500 grams) and 8 pounds, 13 ounces (4,000 grams)
  • High birth weight (HBW). Birth weight of more than 8 pounds, 13 ounces (4,000 grams)

Gestational age:

  • Small for gestational age (SGA). Birth weight less than the 10th percentile for a child born at that gestational age
  • Appropriate for gestational age (AGA). Birth weight from the 10th percentile to the 90th percentile relative to other babies born at that gestational age
  • Large for gestational age (LGA). Birth weight greater than the 90th percentile based on gestational age (also called fetal macrosomia)

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.

Birth weight can predict short and long-term health complications, including chronic disease risk—even among full-term births.

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 Birth Weight

There are many different factors used to determine a baby's birth weight, including age, genetics, and certain lifestyle factors of the mother.

Age

Young mothers (teens) tend to have smaller babies, as do mothers of advanced maternal age (over 35). However, research has also shown a connection between advanced maternal 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

Mothers who smoke 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.

In 2016, 7.2% of women who gave birth reported smoking during pregnancy.

Nutrition

Maternal nutrition can also affect an infant's birth weight. A mother's 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 Maternal (and Paternal) Health

The health of an infant's mother and father can also affect a newborn's birth weight.

  • Mother's weight at conception. Women who are heavier when they become pregnant may have larger babies.
  • Mother's blood sugar and blood pressure. 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 complications. 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.
  • Uterine conditions. Certain hereditary uterine conditions (such as a bicornuate uterus), as well as acquired conditions (fibroids), can result in lower birth weight.
  • Substance abuse. 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, such as:

  • Sex at birth: Male infants tend to weigh slightly more at birth than female infants.
  • Birth order: First babies tend to weigh less than subsequent babies.
  • Multiples: 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.

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.

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