Statisticians on the Birth Weight of Babies

The average or mean birth weight in the United States is approximately 7.5 pounds (3.400 grams). However, average does NOT necessarily mean normal. 

Newborns who are smaller than average might be called small for gestational weight or small for birth weight.

Averages

There are many sizes for babies, ranging from less than 1 pound to more over 16 lbs. 2017 data showed that:

  • 8.28% of babies were low-birthweight. This is defined as less than 5.5 lbs or approximately 2,500g. 
  • 1.4% of babies had very low birth weights (less than 3.3 lbs or 1,500 g). 
  • A full-term infant’s average height was 20 inches.

Sometimes, low birth weight can be appropriate. If a baby is born prematurely (less than 37 weeks gestation), they will “normally” weigh less than 5 pounds (8 ounces) (2500 grams).

Remember that birth weight numbers are calculated using special scales. They are more precise (and regulated than the bathroom scale you use at home) than those you would get from a regular scale. Talk to your pediatrician if you have concerns about your baby’s weight or how they grow.

how many ounces in a pound baby weight

Latest Research

Research has shown that children and adolescents are growing bigger. This is known as the childhood obesity epidemic. Based upon findings from older children and teens it would seem natural to assume that new born are also growing larger.

Statistics show that babies are getting smaller. Research has not demonstrated a direct relationship between lower mean birthweight and an increase of premature babies. There is also no correlation with other factors like more Cesarean deliveries.

Although the cause of the drop in birth weight is not known, it could be due to changes in maternal diet, physical activity and socioeconomic factors.

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 many terms used to describe birth weight. If babies are born prematurely or post-term (overdue), these terms can be confusing.

Instead of using absolute weight, the terms used to describe gestational ages more accurately reflect a baby’s size.

A special growth chart is used for infant classification based on the baby’s gestational weight.

Birth Weight:

  • Very low birth weight (ELBW). Birth weight less than 2 pounds (1,005 g).
  • Very Low Birth Weight (VLBW).. Birth weight less than 3.4 lbs (1,500 g).
  • Low birthweight (LBW). Birth weight less than 5 lbs, 8 ounces (2,500g).
  • Normal birth weight. Between 8 ounces (2,500 grams) and 8 ounces (1,400 grams).
  • High birth weight (HBW). Birth weight greater than 8 pounds, 13 ounces (4,000g).

Gestational age:

  • Small for gestational (SGA). Birthweight less than the 10th percentile of a child born at this gestational stage.
  • Appropriate gestational weight (AGA). Birthweight from the 10th to the 90th percentile relative other babies born at the same gestational time.
  • Large for gestational weight (LGA). Birthweight greater than the 90th percentile based upon gestational time (also known as fetal macrosomia). 

Sometimes, intrauterine Growth Restriction (IUGR) is used to refer to a baby whose birth weight is lower than what was expected for gestational age. It is used most commonly to describe a baby that is growing slower than normal during pregnancy.

Factors that Influence Birth Weight

Many factors are used to determine the baby’s birth weight, including genetics and lifestyle factors.

Age

Young mothers tend to have smaller babies than mothers who are older (over 35). Research has also indicated a link between high birth weight and advanced maternal age.

Genetics

Birth weight is also affected by genetics. Both parents’ genetics are important. However, one difference is that the father’s birth weight has more impact on the mother’s health than it does on the father’s.

Smoking

25 Smoking mothers tend to have smaller babies. This is because the physiological changes caused by smoking decrease the amount of nutrients available to the baby. In 2016, 7.2% of women who gave birth to children reported having smoked during pregnancy.

Nutrition

An infant’s birth weight can be affected by the mother’s nutrition. The factors that influence a mother’s weight gain during pregnancy include her socioeconomic status, pregnancy-related health conditions and genetics.

Prenatal Care

Low birth weight babies have been linked to a lack of regular and early prenatal care. This could be due to poor access to healthcare (for example, limited options in certain areas), mental health concerns or socioeconomic factors.

Overall Maternal (and Paternal) Health

 The health of a baby’s father and mother can have an impact on the newborn’s weight.

  • Mother’s birth weight 
  • Preexisting diabetes is associated with smaller-than-normal babies.Pregnancy complications. Birth weight can also be affected by pregnancy-induced hypertension, gestational diabetic and PIN (high blood sugar during pregnancy). PIN is associated with smaller babies and gestational diabetes is linked to large-for-gestational-age babies.
  • Uterine condition. Certain uterine conditions (such a bicornuate) can lead to a lower birth weight. 
  • Substance abuse. Drug and alcohol use can also impact the baby’s birth weight, usually leading to lower birth weights.

Other Factors

Although many factors can be altered, some cannot.

  • Sex at Birth: Baby boys tend to be slightly heavier than their mothers.
  • Birth Order: First babies are more likely to be smaller than their siblings. 
  • Multiples Twins: Twins, and other multiples, are generally smaller than singletons.

Monitor Newborn Weight Gain

Your baby can be weighed if they are full-term and have a normal birth weight. They don’t usually need to be weighed unless they are eating well, having wet diapers and developing normally.

At each well child visit, your pediatrician will weigh your baby and inform you if there are any concerns. Your pediatrician may ask you to visit the clinic more often if your infant is underweight, prematurely born, or if there are other concerns.

Children born with low birth weights or large gestational ages can experience a variation in their average weight gain. Premature babies are often subject to catch-up growth.

Your pediatrician can explain your growth expectations if your baby was prematurely born. The following guidelines can be used to monitor your baby’s weight growth by age.

The First 3 Months

Baby’s first three months of life are characterized by a growth rate of around 1 inch (or 1.5 to 2 pounds) per month. Your pediatrician will discuss the normal growth rate for young children.

A growth chart can be shown by your doctor. This graph compares your baby’s height to other babies the same age.

4 to 6 months

Babies gain weight slower between the ages of 4 and 6 months. They usually gain 1/2 to 1 inch per month.

From 6 months to 1 year

Between 6 and 9 months old, weight gain slows down. From 6 to 12 months, length growth is approximately 3/8 inch (1 cm) per month. Around one year old, birth weight usually triples.

1- 2 Years

On average, your baby will be about four times their current weight by the age of two and a half. These calculations can be used to determine your child’s adult height .

A Summary

Birth weights of babies can be variable and may be affected by many variables. The baby’s weight at birth does not predict the 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.

Your pediatrician can help you to understand your baby’s growth expectations, regardless of his or her birth weight. You can have graphs created by your pediatrician to show where your baby falls in terms of growth and how that might impact their health.

Sources: 

https://www.apa.org/pi/families/resources/develop.pdf

https://www.elitebaby.us/blogs/news/baby-diaper-size-chart

https://www.elitebaby.us/blogs/news/28-week-baby-premature

https://www.medicalnewstoday.com/articles/325630

How Many Ounces In A Pound Baby Weight