The Size of a Newborn Baby

Your baby grew dramatically when she was still in the womb, but she’s just getting started.

Her size at birth and her growth over time are important indicators of whether she’s thriving. As a result, health care professionals will measure her length, weight and head circumference and chart her growth patterns starting from the moment she is born.

Size at Birth

Newborns come in a broad range of healthy sizes, according to the Nemours Foundation. Most newborns who are born at full-term--between 37 and 40 weeks--weigh between 6 lbs., 2 oz. and 9 lbs., 2 oz. and range in length from 19 to 21 inches. Additionally, the average head circumference for a newborn is about 14 inches, according to Discovery Health.

Size Factors

a lying newborn baby

The Average Length and Weight of Newborns

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A baby’s size at birth is largely affected by the length of the mother’s pregnancy. If a baby is born before his due date, he is likely to weigh less than babies who are born on or after their due dates, according to Nemours. Other factors that contribute to a newborn’s size are the baby’s health and gender, the health of the mother during pregnancy, the size of both parents, whether he was part of a multiple birth and whether he has an older sibling.

Growth Rate

Because babies are born with extra fluid, a healthy newborn typically loses between seven and 10 percent of her birth weight in her first few days, according to Nemours. However, she should gain that weight back by the time she is about two weeks old. In her first month, she should gain about 5 oz. a week and gain about 1 to 1½ inches in height. By the time she reaches five to six months of age, she should be at about double her birth weight, according to the Mayo Clinic.

Smaller and Larger Newborns

a lying newborn baby

The Average Weight of a Newborn

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Babies who weigh less than 5 lbs., 8 oz. at birth are considered “low birth weight” and are at increased risk for health problems, lifelong disabilities and death, according to the March of Dimes. In most cases, these babies are born early or experienced growth restriction in the womb. Newborns who are much larger than average are commonly born to mothers with diabetes, according to Nemours.

These babies may also have special problems, such as trouble regulating blood sugar. Health care professionals observe small and large newborns, and sometimes need to give these babies special medical care before letting the babies go home.

Talking to a Doctor

If you’re concerned about your baby’s growth, talk to her pediatrician. Many perfectly healthy babies go through brief periods where they don’t gain weight or actually lose some weight, according to the Mayo Clinic. Chances are she’s fine, but her pediatrician may be concerned if she gains no weight between well-baby exams.