Low Birthweight
Infants who weigh less than 2,500 grams (or 5 pounds, 8 ounces) at birth are considered to be of low birthweight. Those born weighing less than 1,500 grams (3 pounds, 4 ounces) have very low birthweight. Low birthweight may result from various causes, including premature birth, poor maternal nutrition, teen pregnancy, drug and alcohol use, smoking, or sexually transmitted diseases.
In 2002, 7.8 percent of the 4 million live births that year were low birthweight infants, the highest level reported in more than two decades. About 1.5 percent were very low birthweight infants. Black mothers were about twice as likely as white and Hispanic mothers to have low-birthweight babies (13.4 percent of children born to black birth mothers had low birthweight compared to 6.9 and 6.5 percent, respectively, born to white and Hispanic mothers). (See Table 5.7 and Table 5.8.) Like the proportion of low birthweight babies, the proportion of very low birthweight babies has also been increasing since the 1980s. (See Table 5.9.) The increase in low and very low birthweight babies during the 1990s is attributed to the increase in the multiple birth rate. Babies born as part of a multiple birth are at much greater risk of low birthweight than babies born as a single birth.
Prematurity
The usual length of pregnancy is 40 weeks. Infants born before 37 weeks of pregnancy are considered premature. A premature infant does not have fully formed organ systems. If the premature infant is born with a birthweight comparable to a full-term baby and has organ systems only slightly undeveloped, the chances of survival are great. Premature infants of very low birthweight are susceptible to numerous risks and are less likely to survive than full-term infants. If they survive, they may suffer from mental retardation and other abnormalities of the nervous system.
A severe medical condition called hyaline membrane disease, or respiratory distress syndrome (RDS), commonly affects premature infants. It is caused by the inability of the immature lungs to function properly. Occurring right after birth, it may cause infant death within hours. Intensive care includes the use of a mechanical ventilator to facilitate breathing. Also, premature infants' immature gastrointestinal systems preclude them from taking in
TABLE 5.5
Anencephalus rates per 100,000 live births, 1991–2002
| Year | Anencephalus cases | Total live births | Rate |
| 2002 | 348 | 3,645,770 | 9.55 |
| 2001 | 343 | 3,640,555 | 9.42 |
| 2000 | 376 | 3,640,376 | 10.33 |
| 1999 | 382 | 3,533,565 | 10.81 |
| 1998 | 349 | 3,519,240 | 9.92 |
| 1997 | 434 | 3,469,667 | 12.51 |
| 1996 | 416 | 3,478,723 | 11.96 |
| 1995 | 408 | 3,484,539 | 11.71 |
| 1994 | 387 | 3,527,482 | 10.97 |
| 1993 | 481 | 3,562,723 | 13.50 |
| 1992 | 457 | 3,572,890 | 12.79 |
| 1991 | 655 | 3,564,453 | 18.38 |
| Note: Excludes data for Maryland, New Mexico, and New York, which did not require reporting for anencephalus for some years. | |||
| SOURCE: Adapted from "Figure 2. Anencephalus Rates, 1991–2002," and "Table 2. Number of Live Births with Anencephalus and Rates per 100,000 Live Births for the United States, 1991–2002," in Trends in Spina Bifida and Anencephalus in the United States, 1991–2002, Centers for Disease Control and Prevention, National Center for Health Statistics, Hyattsville, MD, 2004 [Online] http://www.cdc.gov/nchs/products/pubs/pubd/hestats/spine_anen.htm [accessed April 11, 2004] | |||
nourishment properly. Unable to suck and swallow, they must be fed through a stomach tube.
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