In their first year, newborns grow a big way, with most tripling their birth weight and increasing their length by about 50%.

From your baby's first day, health care providers will keep track of weight, length, and head size. Growth is a good indicator of general health, and babies who are progressing well are generally healthy, while poor growth can be a sign of a problem.

What Newborns Weigh

Just like adults, newborns come in a range of healthy sizes. Most full-term babies (born between 37 and 40 weeks) weigh somewhere between 5 pounds 8 ounces (2,500 grams) and 8 pounds, 13 ounces (4,000 grams).

A newborn who is lighter or heavier than the average range is probably perfectly fine but might receive extra attention from the doctors and nurses after delivery just to make sure there are no problems.

A number of things can affect a baby's size at birth. The length of the pregnancy is important. Babies born at their due date or later tend to be larger than those born earlier. Babies born prematurely are often smaller than full-term babies.

Other factors include:

• Size of parents. Tall parents may have larger-than-average newborns; short parents may have smaller-than-average newborns.
• Multiple births. If you're having twins, triplets, or more, you can count on your babies being relatively small. Multiples not only have to share their growing space in the uterus, they also are often born early, which leads to small size at birth.
• Birth order. First babies are sometimes smaller than brothers or sisters born later.
• Gender. Girls tend to be smaller, boys larger, but the differences are slight at birth.
• Mother's health during pregnancy. Factors that can lead to a lower birth weight include a mother's high blood pressure, heart problems, or use of cigarettes, alcohol, or illegal drugs during the pregnancy. If the mother has diabetes, the baby may have a higher birth weight. All conditions that can affect a baby's weight should be closely monitored by the mother's doctor. In addition, women should not smoke, drink alcohol, or use illegal drugs during pregnancy.
• Nutrition during pregnancy. Proper nutrition is essential for a baby's growth in the uterus and beyond. A poor diet during pregnancy can affect how much a newborn weighs and how the infant grows.
• Baby's health. Medical problems, including some birth defects and certain infections acquired during the pregnancy, can affect a child's birth weight and later growth.

Growth and Premature Babies

Premature babies generally are smaller and lighter than other newborns. A preemie's weight will be largely determined by how early he or she was born. The time the infant has missed being in the womb was growing time, so the baby has to do that growing in the outside world.

Many pre-term babies are classified as having "low birth weight" or "very low birth weight." In medical terms, "low birth weight" means a baby weighs less than 5 pounds, 8 ounces (2,500 grams) at birth. That's the case for about 1 in every 12 babies in the United States, so it's quite common. "Very low birth weight" means a baby weighs less than 3 pounds, 5 ounces (1,500 grams). Most babies with low or very low birth weight were born prematurely.

Premature babies are given special medical attention immediately after birth, and a pediatric specialist called a neonatologist may be involved in their care. Many premature babies spend time in the neonatal intensive care unit (NICU) while they receive medical care, including feeding and observation of growth.

Is Bigger Better?

In the old days, a "strapping" baby with chubby cheeks and dimpled thighs was many people's picture of a healthy newborn. But a baby born much larger than average may have special medical problems that need attention.

Some exceptionally large babies, especially those born to mothers with diabetes, including gestational diabetes, may have problems for a few days keeping blood sugar levels up and may require extra feedings, or even intravenous glucose, to prevent those levels from falling too low.

How Newborns Grow

Babies are born with some extra fluid, so it is perfectly normal for a newborn to drop a few ounces when that fluid is lost in the first few days of life. A healthy newborn is expected to lose 7% to 10% of the birth weight but should regain that weight by about 2 weeks after birth.

During their first month, most newborns continue to gain weight at a rate of about one ounce (30 g) per day. They generally grow in height about 1 to 1.5 inches (2.54 to 3.81 centimeters) during the first month. Many newborns go through a period of rapid growth when they are 7 to 10 days old and again at 3 and 6 weeks.

Should I Be Concerned?

Newborns are so small, and it can be hard to know if your baby is gaining weight the way he or she should. You may worry that your baby has lost too much weight in the first few days or isn't taking enough breast milk or formula. Most likely, everything is fine. But if you are concerned at all, check with your doctor.

What's Next?

Being small or large at birth doesn't necessarily mean a baby will be small or large later in childhood or as an adult. Plenty of towering teenagers began life as small babies, and the biggest baby on the block can grow up to be a petite adult.

By the time they're adults, kids tend to resemble their parents in size. Genetics, as well as good nutrition and your attention, will play a large part in determining how your baby grows in the years to come.

Whether your baby starts out large, small, or in between, in the next few months you can expect your infant to keep growing fast.

Reviewed by: Mary L. Gavin, MD
Date reviewed: September 2011