Baby Birth Weight Statistics

Average, normal, small, and large babies

The mean or average birth weight in the United States is 7 lb, 7.5 oz (3,389 grams), but average doesn't mean "normal." A birth weight of a term newborn between 5 lbs, 8 oz (2,500 grams), and 8 lbs, 13 oz (4,000 grams) is considered to be normal. 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, large for gestational age.

There are many factors that affect birth weight, including genetics, prenatal care, maternal nutrition, smoking, and medical conditions. Let's look at the terminology and meaning of weight at birth, and how you can expect your baby to grow in weight and length during the first year of life.

Baby Birth Weight Statistics

In 2016, the last year for which final birth data are available in 2018, there were 3,853,472 babies born. Although most babies are born between 6 lbs, 9 oz (3,000 grams) and 7 lbs, 11oz (3,499 grams), the range of "normal" is defined as being from 5 pounds, 8 ounces (2,500 grams) to 8 lbs, 13 oz (4,000 grams). That said, babies come in many sizes, from just under 1 pound to more than 16 pounds.

In 2016, 8.17 percent of babies were considered to be low birth weight, or less than 5 lbs, 8 oz (2,500 grams), and 1.4 percent of babies were very low birth weight (less than 3.3 lbs or 1,500 grams). Of note is that low birth weight may be appropriate, for example, a baby born at 30 weeks gestation would "normally" weigh less than 5 lbs, 8 oz.

On the other side of the scale, roughly 9 percent of babies are large for gestational age at birth (see definitions below).

The average length of a full-term infant 20 inches, with a normal range of 18 inches to 22 inches.

Birthweight Terminology

There are a number of different terms that are used to describe birth weight and this can be confusing, especially with babies who are born preterm or postdates (overdue). Instead of using absolute weight, such as with low birth weight above, terms that describe gestational age more accurately reflect if a baby's size is small, normal, or large based on gestational age at birth. 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 lbs (1000 grams)
  • Very low birth weight (VLBW): Birth weight less than 3.3 lbs (1500 grams)
  • Low birth weight (LBW): Birth weight less than 5.5 lbs (2500 grams)
  • Normal birth weight: Between 5 lbs, 8 oz (2,500 grams) and 8 lbs, 13 oz (4000 grams)
  • High birth weight (HBW): Birth weight of more than 8 lbs, 13 oz (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 is most often used to describe babies who are growing less than expected during pregnancy.

Why all of the different classifications? Many 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 2,500g (5.5 pounds) would likely be classified as being both SGA and IUGR.

Baby Birth Weight Trends

It is well known that kids are getting bigger, with the childhood obesity epidemic continuing to be a problem. And some experts now think that some of the obesity problems can start as early as the newborn period. So are newborn babies getting bigger, too?

Recent trends in average birth weight include:

  • 1990 - 7 lbs, 9.4 oz
  • 1995 - 7 lbs, 9.17 oz
  • 2000 - 7 lbs, 8.95 oz
  • 2005 - 7 lbs, 7.54 oz

Statistics show that babies are actually getting a little smaller, and this is not thought to be due to more premature babies or other independent factors.

Weight Measurements

It's important to note that the birthweight 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 babies weight either at birth or later on, make sure to get an accurate reading at your clinic.

Factors Affecting Birthweight

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

Mother's 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 woman 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, in turn, is sometimes a result of poor access to health care due to geographic location, mental health concerns, or socioeconomic conditions.

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 maternal history of high blood pressure (before 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

  • 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 5 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 she has any concerns. 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.

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 days 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 Months 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 year to 2 Years

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

Exceptions

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.

A Word From Verywell

Birth weight of babies can vary widely and is affected by many factors. Of note is that 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. Healthy lifestyle factors, such as not smoking and eating a healthy diet can play a role in an infants weight, but factors that can't be controlled, such as genetics, medical conditions, and even gender are important as well.

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