• In the United States, just under 90% of babies are born full term, healthy, and happy. Approximately 12% of women give birth to babies who are born too early (premature). A normal pregnancy lasts between 38 and 40 weeks (measured from the first day of a woman's last menstrual period). Babies who are born before a woman has completed her 37th week of pregnancy are considered premature. Approximately 8% of babies are born too small (low birth weight). Babies who are born too small are divided into three groups:
  • low birth weight babies-these babies weigh between 3 pounds, 5 ounces and 5 pounds, 8 ounces (1,500 to 2,500 grams) at birth;
  • very low birth weight babies-these babies weigh between 2 pounds, 3 ounces and 3 pounds, 5 ounces (1,000 to 1,500 grams) at birth; and
  • extremely low birth weight babies-these babies weigh less than 2 pounds, 3 ounces (or 1,000 grams) at birth.

A baby that is born prematurely often has a low birth weight, but not always. A baby may be premature and be normal weight or full-term and be low birth weight.

Table 1 below shows the number of babies born in the U.S. in 1997, according to the baby's gestational age. (Gestational age refers to the number of weeks a woman is pregnant. It is usually calculated from the first day of the last normal menstrual period.)

Table 1: Number and Percent of Live Births in the United States by Gestational Age (1997)
Weeks GestationNumber of BirthsPercent of Total Live Births
Less than 28 weeks28,0050.7%
28-31 weeks46,3981.2
32-35 weeks204,2325.3
36 weeks157,9654.1
37-39 weeks1,793,42146.2
40 weeks851,72921.9
41 weeks458,14511.8
42 weeks or greater302,5417.8
Unknown38,4581.0
TOTAL 3,880,894100.0%

Note: This table is adapted from Anderson's Delivery of the Preterm Infant.

Common Problems of Prematurity

Premature babies often have medical problems because their organs and body systems have not developed sufficiently. The lungs, brain, gastrointestinal tract, and immune system are most often affected and pose the greatest risks for causing long-term problems and death (see the tables below). To some degree, all organs and body systems are immature and may cause complications in the premature baby.

Table 2: Complications of Very Premature Babies, by Birth Weight
Birth Weight:501-750 gms

(1.1-1.7 lbs)
751-1,000 gms

(1.7-2.2 lbs)
1,001-1,250 gms

(2.2-2.8 lbs)
1,251-1,500 gms

(2.8-3.3 lbs)
Complication:

Gestational age (weeks)
24-26 26-28 28-30 30-32
Respiratory distress syndrome71%54%36%22%
Moderate bleeding in the brain (Grade 3)12%9%6%4%
Severe bleeding in the brain (Grade 4)13%5%3%1%
Chronic lung disease47%33%15%6%
Inflammation or death of part of the GI tract (necrotizing enterocolitis, or NEC)11%9%6%3%
General infection44%30%17%7%
Survival to 120 days or hospital discharge56%88%94%96%
Survival without any of these complications36%59%79%90%

Note: Table contains data from the National Institute of Child Health and Human Development, Neonatal Research Network for 1997-2000, adapted from Avery's Diseases of the Newborn, 8th Edition.

The Neonatal Intensive Care Unit (NICU)

Because of these problems of prematurity, many babies who are born too small or too early need special care in a neonatal intensive care unit (NICU). As might be expected, the smaller or more premature a baby is, the more likely it is to need special care. For example, babies with very low birth weight make up the majority of babies in the NICU. Babies with extremely low birth weights, however, make up a much smaller number of the babies in the NICU but they stay longer-as a group, they account for 40% of the days spent per year in the average NICU.

The NICU has made a big difference in the outcomes of premature and low birth weight babies. Twenty years ago, most babies who weighed between 500 and 1,000 grams (1 pound, 2 ounces and 2 pounds, 3 ounces, respectively) at birth would survive only a few hours to a few days. At present, the majority of these babies survive, but they may be in the NICU for 30 to 90 days, depending on their birth weight.