Baby Birth Weight Statistics

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

The mean or average birth weight in the United States is 7 pounds, 7.5 ounces (3,389 grams), but average doesn't mean normal. A birth weight between 5 pounds, 8 ounces (2,500 grams) and 8 pounds, 13 ounces (4,000 grams) is considered normal for a full-term newborn. Babies that are smaller than this may be referred to as low birth weight or small for gestational age, and babies that are larger than this are considered to be large for gestational age.

The Statistics

There were 3,853,472 births registered in 2016 in the United States, according to National Vital Statistics Reports published by the Centers for Disease Control and Prevention (CDC).

In the United States, state laws require birth certificates to be completed for all births. Federal law mandates the collection and publication of all births and other vital statistics data, which is compiled by The National Vital Statistics System to provide statistical information from birth certificates. The following can be gleaned from the data:

Averages

Although babies come in many sizes, from just under 1 pound to more than 16 pounds, the 2106 data shows:

  • The average weight at birth was between 6 pounds, 9 ounces (3,000 grams) and 7 pounds, 11 ounces (3,499 grams).
  • 8.17 percent of babies were considered to be low birth weight, which is defined as less than 5 pounds, 8 ounces (2,500 grams).
  • 1.4 percent of babies were very low birth weight (less than 3.3 pounds, or 1,500 grams).
  • Roughly 9 percent of babies are large for gestational age at birth (learn more below).
  • The average length of a full-term infant is 20 inches.
  • The normal range for full-term infants is 18 inches to 22 inches.

It is important to note that in some cases, low birth weight may be appropriate. For example, a baby born at 30 weeks gestation would "normally" weigh less than 5 pounds, 8 ounces.

In addition, the birth weight numbers above are derived from special scales that are much more accurate and regulated than your bathroom scale at home. If you are concerned at all about your baby's weight either at birth or later on, be sure to get an accurate reading at your clinic.

Interesting Trends

Research has shown that kids and adolescents are getting bigger as the childhood obesity epidemic continues to be a problem. Due to those findings, it might seem only natural to assume that newborn babies are also getting bigger.

However, the statistics show that babies are actually getting a little smaller, and the finding is not thought to be due to more premature babies or other independent factors such as more Cesarean births. The actual cause of the decline is unknown and may 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.
  • 1995 - 7 lbs., 9.17 oz.
  • 2000 - 7 lbs., 8.95 oz.
  • 2005 - 7 lbs., 7.54 oz.

Terminology

There are a number of different terms that are used to describe birth weight, which can be confusing, especially with babies who are born preterm or postdates (overdue).

Instead of using absolute weight, terms that describe gestational age more accurately reflect a baby's size.

Depending on their weight at birth and their gestational age, using special growth charts, babies are typically classified as being:

  • 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)
  • Extremely low birth weight (ELBW): Birth weight less than 2.2 pounds (1000 grams)
  • Very low birth weight (VLBW): Birth weight less than 3.3 pounds (1500 grams)*
  • Low birth weight (LBW): Birth weight less than 5.5 pounds (2500 grams)*
  • Normal birth weight: Between 5 pounds, 8 ounces (2,500 grams) and 8 pounds, 13 ounces (4000 grams)
  • High birth weight (HBW): Birth weight of more than 8 pounds, 13 ounces. (4000 grams)

*The term intrauterine growth restriction (IUGR) may also be used to describe a baby with a birth weight lower than expected for gestational age, but it is most often used to describe babies who are growing less than expected during pregnancy.

Why All of the Different Classifications?

These classifications are useful because they often correspond to methods in which clinical care and treatment are provided.

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 his gestational age. On the other hand, a full-term baby who was 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 that play a role in a baby's birth weight, including:

Age

Young mothers (teens) tend to have smaller babies, whereas mothers over the age of 35 have larger babies on average.

Genetics

Genetics play a role in birth weight, with characteristics of both parents being 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 percent of women reported smoking during pregnancy.

Nutrition

Maternal nutrition plays an important role in birth weight. A mother's weight gain in pregnancy, can, in turn, be influenced by a number of different factors including socioeconomic conditions, pregnancy, and non-pregnancy related health conditions, and genetics.

Prenatal Care

A lack of early and regular prenatal care is associated with lower birth weight babies. Lack of care is sometimes a result of poor access to health care due to geographic location, mental health concerns, or socioeconomic conditions.

Overall Maternal (and Paternal) Health

The health of both the mother and father can impact birth weight of the baby. Some health-related factors include:

  • Mother's weight at conception: Mothers who are heavier when they become pregnant tend to have larger babies.
  • Mother's blood sugar and blood pressure: A woman's history of high blood pressure before her child's birth is associated with smaller babies, whereas 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) likewise affect birth weight, with PIN being associated with smaller babies and gestational diabetes linked with large-for-gestational-age babies.
  • Uterine conditions: There are a number of hereditary uterine conditions (such as a bicornuate uterus) as well as acquired conditions (fibroids) that may result in lower birth weight due to a smaller area in the womb for the baby to grow.
  • Substance abuse: Alcohol use, as well as abuse of other controlled substances, can affect the weight of a baby.

    Other Factors

    While many of the above factors are things that can be modified, there are additional ones than cannot:

    • Sex: Boys tend to weigh slightly more at birth than girls.
    • Birth order: First children tend to weigh less than subsequent children, with weight increasing, on average, by 4 ounces for each baby up to five babies.
    • Multiples: Twins and other multiples are usually smaller than singletons.
    • Race: Caucasian babies tend to be heavier than Asian, African-American, or Native-American babies.

    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 she is eating well, has wet diapers, and is acting normally. Your pediatrician will check her 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. Premature babies often undergo catch-up growth, and your pediatrician can talk to you about expectations if your baby was born early.

    Otherwise, weight gain by age can be monitored with the following guidelines in mind:

    Initial Weight Loss

    After delivery, babies usually lose weight at first. This weight loss is roughly 5 percent of body weight in babies who are bottle-feeding and 7 percent to 10 percent in babies who are breastfeeding. Babies usually regain their birth weight by 10 to 14 days of age.

    The First 3 Months

    During the first 3 months, babies gain an average of 1.5 pounds and grow an average of 2 centimeters (around 1 inch) per month. Your pediatrician will talk about normal growth rates for young children and will show you how your baby is doing on a growth chart, a graph that compares her height and weight to other babies of the same age.

    3 to 6 Months

    Between the third and the sixth month, babies gain roughly 1 pound and grow roughly 1 centimeter per month. Birth weight is usually doubled around the 5-month mark.

    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 1/2 centimeter per month from 6 months to 12 months. Birth weight is usually tripled by around 1 year of age.

    1 to 2 Years

    Your babies weight will roughly quadruple by the age of 2 years. At this point in age, there are calculations that can be made that can estimate adult height.

    A Word From Verywell

    Birth weight of babies can vary widely and is affected by many factors. Birth weight doesn't necessarily predict a baby's adult size, and some very-low-birth-weight babies can end up very large, while some large-for-gestational-age babies can end up quite small as adults. What's most important is that you have a clear sense of where your child falls in terms of expectations and how that might affect her health.

    Was this page helpful?
    Article Sources