Although 37 weeks gestation is considered mature, a fetus can reach maturity as early as 35 weeks. This does not mean that younger or smaller babies cannot survive. Today, they frequently do. With neonatal intensive care, highly sophisticated equipment for assisting babies to breathe, a recently developed medication called surfactant, which helps the lungs stay expanded, and round-the-clock attention by expert staff, premature babies have a better chance of survival than they did a few decades ago. Newborns can survive with birth weights as low as 500 grams(just over one pound) and a gestational age of about 23 or 24 weeks. Such tiny infants, however, often have developmental problems in childhood.

Most of the difficulty in functioning among preterm babies is seen in babies less than 34 weeks gestational age. Government statistics today make a distinction between moderately preterm births(32 to 36 weeks) and very preterm births(less than 32 weeks completed gestation). Distinctions are also made among low birth weight(less than 2,500 grams or 5½ pounds), very low birth weight(1,500 grams or less, approximately 31/3; pounds), and extremely low birth weight(1,000 grams or less, approximately 2¼ pounds).

In 1998, the overall percentage of premature births in the U.S. was 11.6. This rate has been on the rise. In 1990, it was 10.6 percent; in 1981, 9.4 percent. Most of the increase in 1998 was in the moderately preterm group-babies born between 32 and 36 weeks gestation.

One major reason that preterm births are rising is that multiple births have become more common. Pregnancies with more than one baby are more likely to end prematurely than are pregnancies with only one baby(called singleton pregnancies). Multiple birth babies are also more likely to be low birth weight.

Two trends account for the recent increase in multiple births. One is the greater number of births to women in their thirties, who are naturally more likely to have a multiple birth than younger women. The other is the proliferation of fertility treatments, some of which result in multiple fetuses. About 80 percent of births of triplets or more were due to fertility treatment in 1996 and 1997. In 2000, 18 percent of births to women aged 45 to 49 years was a twin, triplet, or higher order multiple birth. In 1999, one out of every 3 births to women aged 50 or older was a twin or triplet or higher-order multiple birth.

We cannot discuss the rate of prematurity in the United States without noting the difference between the rate for Caucasian women(10.2 percent) and for African-American women(17.6 percent). A small part of this difference is due to the slightly more common occurrence of multiple births among African-American women. Socioeconomic background (poverty, inadequate nutrition, and lack of access to health care) accounts for another part of the difference. Even among African-Americans who have achieved a high socioeconomic status, however, there is a difference in the preterm birth rates compared with Caucasians. We can only reflect that racial inequalities in this country continue to have effects even when aspects of inequity are overcome.

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