GROWTH AND YOUR NEWBORN

"It's a boy!"
"It's a girl!"

Once that's been established, what's the next thing you'll find out about your newborn? Probably weight and length. And when sending out your birth announcements, you'll probably include both of these measurements. Why all this interest in your baby's size? Because it's a concrete way to begin tracking your baby's growth from the first day forward.

How Much Should My Baby Weigh?

Just like adults, newborns come in all shapes and sizes. The majority of babies born full-term (37 to 40 weeks) weigh anywhere from 6 pounds to 9 pounds. Babies that weigh less than 5 pounds, 8 ounces are considered to have low birth weight. Length is usually between 19 and 21 inches long.

There are many factors that affect a baby's size at birth, including:

Size of parents. It's not unusual for tall parents to have larger-than-average newborns or for shorter people to have smaller-than-average newborns.
Baby's gender. Girls tend to be slightly smaller, boys slightly larger.
Length of pregnancy. Babies born around the mother's due date and later tend to be larger than those born earlier.
Multiple births. If you're having twins, triplets or more, you can count on each baby being relatively small, since they have to share the space in the womb and are often born early.
Mother's health during pregnancy. Certain maternal illnesses can affect the weight of a newborn. For example, high blood pressure can lead to a lower birth weight, while diabetes can contribute to a higher birth weight. All conditions that can affect a baby's weight should be closely monitored by the mother's doctor.
Mother's nutrition during pregnancy. Proper nutrition during the nine months of pregnancy 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 a baby grows.
Mother's lifestyle choices during pregnancy. Smoking, drinking or using illegal drugs can all have a significant negative effect on a baby's birth weight.
Birth order. First babies are sometimes smaller at birth than subsequent babies. Your newborn will most likely gain weight at a rate of approximately 2/3 of an ounce per day and grow about 1 to 1.5 inches during his first month. Most newborns go through a period of rapid growth (a growth "spurt") when they are seven to 10 days old and again between three and six weeks.

Should I Be Concerned?

If your full-term newborn is lighter or heavier than average, he or she will likely receive extra attention after birth to make sure there are no special problems.

Very small babies tend to have more trouble feeding and keeping their body temperature regulated. Mothers of smaller babies may be encouraged to nurse (or bottle-feed) frequently, and these babies' temperatures may be monitored closely.

Although our typical picture of a healthy newborn often includes chubby cheeks and thighs, a baby born much larger than normal may have special medical problems that need attention. Some exceptionally large babies may have problems with blood-sugar levels and may require extra feedings to prevent those levels from falling too low.

Premature babies generally are smaller and lighter than their full-term counterparts. A preemie's weight will be largely determined by exactly how early he or she is born. The time that has been missed in the womb was growing time, so he or she will have to do that growing in the outside world. Premature babies are given special medical attention immediately after birth, and a pediatric specialist called a neonatologist may be involved in care.

Premature babies have little body fat to keep them warm, so until they build up their body weight, they may be placed in special enclosed cribs where the temperature can be adjusted. All of a preemie's nourishment, whether from mother's breast milk or formula, will be closely monitored to make sure the infant is getting enough nourishment to help growth progress smoothly.

What's Next?

Don't be worried if your full-term baby goes home weighing slightly less than at birth. While it's disappointing for parents to see their newborns drop ounces, it's perfectly normal. Nearly all newborns lose a portion of their birth weight because of fluid loss and because they may be unable to eat very much during the first few days of life. By day 10 or before, your baby should be back at his or her birth weight and by four months most babies double their birth weight. If you are concerned about what seems to be an excessive weight loss after the first few days of your baby's life, check with your baby’s doctor.

On a final note, neither smaller nor larger newborns are sure to become smaller or larger children or adults. Genetics will play a large part in determining the speed at which your baby grows after birth ... although, in a parent's eyes, they all seem to grow so fast!

Last Update: July 2008, by Martin Lipsky, M.D.