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

How Big Are Newborns?

Newborns come in a range of healthy sizes. Most 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).

Newborns who are lighter or heavier than the average baby are usually fine. But they might get extra attention from the doctors and nurses after delivery to make sure there are no problems.

Different things can affect a baby's size at birth. The length of the pregnancy is important. Babies born around their due date or later tend to be larger than those born earlier.

Other factors include:

• Size of parents. Big and tall parents may have larger-than-average newborns; short and petite parents may have smaller-than-average newborns.
• Multiple births. If you have twins, triplets, or more, you can count on your babies being a bit small. Multiples have to share their growing space in the uterus, and they're 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.
• Mom's health during pregnancy. Things that can lead to a lower birth weight include a mother with high blood pressure or heart problems; or one who used cigarettes, alcohol, or illegal drugs during the pregnancy. If the mother has diabetes or is obese, the baby may have a higher birth weight.
• Nutrition during pregnancy. Good nutrition is vital for a baby's growth — before and after birth. A poor diet during pregnancy can affect how much a newborn weighs and how the infant grows. Gaining a lot of weight can make a baby more likely to be born bigger than average.
• Baby's health. Medical problems, including some birth defects and some infections during the pregnancy, can affect a child's birth weight and later growth.

What About Preemies?

Premature babies generally are smaller and weigh less than other newborns. A preemie's weight will largely depend on how early he or she was born. The time an infant missed being in the womb was growing time, so the baby has to do that growing after birth.

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 birth weight or very low birth weight were born prematurely.

Premature babies get special medical attention right away after they're born. A specialist called a neonatologist may help care for them. Many preemies spend time in the neonatal intensive care unit (NICU) while they get medical care.

Is Bigger Better?

A baby with chubby cheeks and dimpled thighs once 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 very large babies — especially those born to mothers with diabetes, including gestational diabetes — may have problems for a few days keeping blood sugar levels up. They might need extra feedings or even IV (given into a vein)  glucose to keep those levels from falling too low.

Will My Baby Lose Weight?

Yes, at first. Babies are born with some extra fluid, so it's normal for them to drop a few ounces when they lose that fluid 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 within the first 2 weeks or so after birth.

During their first month, most newborns gain weight at a rate of about 1 ounce (30 grams) per day. They generally grow in height about 1 to 1½ 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. If so, talk to your doctor, who may ask you about:

• How many feedings a day your baby gets. A breastfed baby may feed about 8 or more times in a 24-hour period; formula-fed babies usually eat less often, perhaps every 3 to 4 hours. A lactation (breastfeeding) counselor can make suggestions to increase comfort and improve technique, if a mom needs extra help.
• How much your baby eats at each feeding. A baby generally nurses for at least 10 minutes, should be heard to swallow after 3 or 4 sucks, and should seem satisfied when done. At this age, formula-fed babies may drink up to 3 to 4 ounces (90 to 120 milliliters) at a time.
• How often your baby pees. A breastfed baby may have only 1 or 2 wet diapers a day until the mother's milk comes in. Expect about 6 wet diapers by 3 to 5 days of age for all babies. After that, babies should have at least 6 to 8 wet diapers a day.
• How many bowel movements your baby has each day, and what they're like. Newborns may have only one poopy diaper a day at first. Poop is dark and tarry the first few days, then becomes soft or loose and greenish-yellow by about 3 to 4 days. Newborns usually have several poopy diapers a day if breastfed and fewer if formula-fed.

What Else Should I Know?

Being small or large at birth doesn't mean a baby will be small or large later in childhood or as an adult. Plenty of tall teens began life as small babies, and the biggest baby in the family 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 how your baby grows in the years to come.

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