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Low Birthweight -- United States, 1975-1987

The incidence of low birthweight (LBW) is monitored in the United States because it is an important indicator of infant morbidity and mortality. This report highlights findings on trends in LBW in the United States from 1975 through 1987 (the most recent year for which data are available) (1). These findings are based on analysis of birth certificate data provided by the 50 states and the District of Columbia to CDC's National Center for Health Statistics. For each birth, data include birthweight and related demographic and health information for the mother and infant.

From 1975 through 1985, the incidence of LBW ( less than 2500 g ( less than 5 lbs. 8 oz.)) declined from 73.9 per 1000 live births to 67.5 per 1000, a 9% decrease (Table 1). However, LBW increased 2.2% from 1985 through 1987. Moderately low birthweight (MLBW) (1500-2499 g (3 lbs. 4 oz. to 5 lbs. 8 oz.)) declined by 11% from 1975 through 1985 but also increased 2.2% from 1985 through 1987. Very low birthweight (VLBW) ( less than 1500 g ( less than 3 lbs. 4 oz.)) increased by 4% from 1975 through 1985 and increased another 2.5% from 1985 through 1987. Most of the decline in LBW and MLBW occurred before 1980 (86% and 78%, respectively); all the increase in VLBW occurred after 1980.

Although LBW declined for both white infants and black infants before 1980, the decline was nearly twice as great for white (9%) as for black infants (5%) (Table 1). The decline in LBW rates in the first half of the 1980s was less than or equal to 1% for both white infants and black infants. During the same time, the increase in VLBW was more than twice as great for black (9%) as for white (4%) infants. From 1985 through 1987, LBW rates increased by slightly less than 1% for white infants and by 2% for black infants; the incidence of VLBW for white infants was stable but rose an additional 3% for black infants (Table 1).

In 1985, 52% and 93% of MLBW and VLBW infants, respectively, were born preterm ( less than 37 weeks of gestation) (Table 2). From 1981 through 1985, the rate for full-term LBW infants declined by 7%, but the rate for preterm LBW infants increased by 2% (1). Thus, the small decline in the overall rate of LBW in this period is due entirely to the reduction in the rate of full-term LBW infants.

In 1985, compared with a longer interbirth interval (2-4 years after the previous live birth), a short interbirth interval (1-1 years after the previous live birth) was associated with a two-thirds greater likelihood of LBW and an approximately 80% greater likelihood of VLBW (Table 3). Reported by: Div of Vital Statistics, National Center for Health Statistics; Div of Reproductive Health, Center for Chronic Disease Prevention and Health Promotion, CDC.

Editorial Note

Editorial Note: The data in this report underscore the substantial and persistent difference between black and white infants in the risk for LBW. In 1975, black infants were 2.1 times as likely as white infants to have a birthweight of less than 2500 g. Because the LBW rate declined slightly more for white than for black infants from 1975 through 1985, the relative risk for black infants increased to 2.2 by 1985 and remained at this level through 1987. The relative risk of VLBW for black infants also increased (from 2.6 in 1975 to 2.9 in 1987) (1).

Reasons for the worsening gap between rates for black and white LBW and VLBW infants are complex. Relatively more black than white mothers are represented in subgroups at high risk for LBW (i.e., unmarried, less than 20 years of age, with less than 12 years of education, or with late or no prenatal care). Within each subgroup, however, black mothers are generally twice as likely to have LBW infants and two to three times as likely to have VLBW infants (1). Increased maternal education lowers the risk for LBW for both black and white infants in high-risk categories; however, the risk for black infants relative to white infants in LBW incidence actually increases with added years of completed education (1). At comparable levels of education, black mothers have a lower average family income than do white mothers (2). These socioeconomic differences may affect the quality of health care available to black women.

Other factors related to the higher rates of LBW among black infants include poorer nutritional status among black mothers, higher rates of mistimed pregnancies, and higher rates of unwanted births (3). Black women are generally more likely than white women of similar prepregnancy weight to gain less than 16 lbs. during their pregnancy, and this lower weight gain is associated with an increased risk for LBW (4). Anemia is associated with preterm delivery; an estimated 5.0%-8.3% of preterm deliveries among black mothers above the preterm deliveries among white mothers is due to excessive rates of anemia among black women (5,6). Iron supplementation for pregnant women with borderline or frank anemia should lead to a modest reduction in preterm delivery and in the relative risk of LBW among black infants (6).

Consistent with the finding that infants born less than 2 years after a previous child are at greater risk for LBW, infants who are either unwanted or conceived before the mother is ready to bear another child have a greater risk for LBW (3). The proportion of LBW attributable to mistimed or unwanted births among black infants has been estimated at greater than 16% (6). Regardless of whether this attributable risk is causal or is related to other factors associated with unplanned pregnancies, the prevention of unintended pregnancies could substantially reduce the difference in rates of LBW between blacks and whites (6).

Recent recommendations on improving prenatal care (7) specify that care should begin before conception and should include pregnancy planning; involvement of a care coordinator; and comprehensive treatment for all identified risks, including behavioral and nutritional factors. The provision of comprehensive, coordinated prenatal care has been associated with reduced risk for LBW among poor, predominantly black prenatal patients (8).

New information relevant to the etiology of LBW will be available for 1989 from the revised U.S. Certificate of Live Birth for 47 states and the District of Columbia. The revised certificate includes questions relating to medical risk factors during pregnancy, such as anemia and cardiac disease, and such factors as tobacco and alcohol use and weight gain during pregnancy that are closely associated with birthweight. These data, combined with other socioeconomic and health data from birth certificates, should help clarify the reasons for the persistent and large racial differentials in the incidence of LBW and infant mortality (9).

References

  1. Taffel SM. Trends in low birth weight: United States, 1975-85. Vital Health Stat 1989:21(48); DHHS publication no. (PHS)89-1926.

  2. Kleinman JC, Kessel SS. Racial differences in low birth weight. N Engl J Med 1987;317:749-53.

  3. Pamuk ER, Mosher WD. Health aspects of pregnancy and childbirth. Vital Health Stat 1988:23(16); DHHS publication no. (PHS)89-1992.

  4. Taffel SM. Maternal weight gain and the outcome of pregnancy, United States, 1980. Vital Health Stat 1986:21(44); DHHS publication no. (PHS)86-1922.

  5. Klebanoff MS, Shiono PH, Berendes HW, Rhoads GG. Facts and artifacts about anemia and preterm delivery. JAMA 1989;262:511-5.

  6. Hogue CJR, Yip R. Preterm delivery: can we lower the black infant's first hurdle? JAMA 1989;262:548-50.

  7. Expert Panel on Content of Prenatal Care. Caring for our future: the content of prenatal care. Washington, DC: US Department of Health and Human Services, Public Health Service, 1989.

  8. Buescher PA, Smith C, Holliday JL, Levine RH. Source of prenatal care and infant birth weight: the case of a North Carolina county. Am J Obstet Gynecol 1987;156:204-10.

  9. Taffel SM, Ventura SJ, Gay GA. Revised U.S. certificate of birth--new opportunities for research on birth outcome. Birth 1989;16:188-93.

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