Increasing Incidence of Low Birthweight -- United States, 1981-1991

A national health objective for the year 2000 is to reduce low birthweight (LBW) (less than 2500 g {less than 5 lbs, 8 oz}) to an incidence of no more than 5% of live-born infants (50.0 per 1000) (objective 14.5) (1). During 1970-1985, the incidence of LBW in the United States declined steadily (2); however, from 1985 to 1991, the incidence increased slightly, from 67.5 to 71.2 (2,3). In 1991, disorders relating to short gestation and LBW were the primary cause of death among black infants and the third leading cause among white infants (4). To characterize trends in the race-specific incidence of LBW by period of gestation from 1981 to 1991, data from birth certificates were analyzed. This report summarizes the results of that analysis.

Data were derived from birth certificates for live-born U.S. infants during 1981-1991. For this analysis, LBW infants were categorized as term-LBW (greater than or equal to 37 completed weeks gestation) and preterm-LBW (less than 37 completed weeks gestation). The date of last normal menstrual period (LMP), the basis for computing the period of gestation, was reported by 49 states and the District of Columbia (DC) from 1981 through 1984 and all states and DC from 1985 through 1991. Weeks of gestation were imputed only when the day of LMP was missing. During 1989-1991, the clinical estimate of gestational age was used when month and/or year of LMP were missing or when gestational age based on date of LMP was not compatible with birthweight. During 1981-1988, approximately 4% of births were excluded from the analysis because of missing data; during 1989-1991, 1.0%-1.5% of births were excluded. Because both demographics and underlying risk factors for LBW vary by race (2,5), the analysis was stratified by race of mother. Data are presented only for blacks and whites because of the small number of births to women of other races.

From 1981 to 1991, the incidence of LBW for infants with known gestation increased 6.6%, from 66.4 per 1000 live-born infants in 1981 to 70.8 in 1991 (Table_1). The rate of term-LBW infants decreased 8.6%, from 29.0 to 26.5; for both black and white infants, the rate of term-LBW infants decreased 9.8% (from 52.3 and 24.4, respectively, to 47.2 and 22.0, respectively,). However, the rate of preterm-LBW infants increased 18.1%, from 37.5 to 44.3; for black infants, the rate of preterm-LBW infants increased 21.6% (from 72.1 to 87.7) and for white infants, 15.2% (from 31.0 to 35.7) (Table_1).

Changes occurred in the distributions of selected maternal (i.e., age, marital status, and receipt of prenatal care) and infant (i.e., singleton status) characteristics during 1981-1991 that can affect birthweight (6) (Table_2). Among women aged greater than or equal to 35 years, the percentage of births increased 100% (from 4.7% in 1981 to 9.4% in 1991); among women who were unmarried, 58% (from 18.6% in 1981 to 29.4% in 1991); and among women who had received no prenatal care, 50% (from 1.2% in 1981 to 1.8% in 1991). Nonsingleton births (e.g., twins) increased 20% (from 2.0% in 1981 to 2.4% in 1991). The direction of trends was similar for both blacks and whites; however, the magnitude varied by race. For example, the percentage of births among women who had received no prenatal care increased 1.6 times more rapidly for black women than for white women.

To control for the changing distributions from 1981 to 1991, incidences of LBW for both years were directly standardized by using the combined 1981 and 1991 population distributions of maternal age, marital status, receipt of prenatal care, and infant's singleton status (Table_3). Combined, the changes in the distributions of maternal and infant factors explained 68.0% of the increase in incidence of preterm-LBW infants for white women and 42.9% of that for black women. The change in the distribution of maternal age alone explained few or none of the LBW trends for either race.

Reported by: Div of Nutrition, National Center for Chronic Disease Prevention and Health Promotion; Div of Vital Statistics, National Center for Health Statistics, CDC.

Editorial Note

Editorial Note: The findings in this report indicate that the increase in incidence of LBW from 1981 to 1991 resulted from the increase in preterm-LBW infants. Compared with infants of normal birthweight (greater than or equal to 2500 g {greater than or equal to 5 lbs, 8 oz}), LBW infants are five to 10 times more likely to die within the first year of life; furthermore, preterm-LBW infants are approximately three times more likely to die than term-LBW infants (7).

The findings in this analysis are subject to at least one limitation -- the change to include clinical estimates in the computation of gestational age in 1989. The greatest increase in the incidence of preterm-LBW infants occurred that year. However, when these estimates were removed from the computations for 1991, the increase in incidence was reduced 13% for all infants, 9% for white infants, and 6% for black infants. Thus, 87%-94% of the increases in the incidence of preterm-LBW infants from 1981 to 1991 were unrelated to the inclusion of clinical estimates in the computation of gestational age.

The etiology of term-LBW infants and preterm-LBW infants differs (8). For term-LBW infants, most underlying causes (e.g., maternal smoking, weight at conception, and gestational weight gain) have been identified; for preterm-LBW infants, the etiology largely remains unexplained (6,8). In the United States, the increase in incidence of preterm-LBW infants during 1981-1991 reflects in part changes in the distribution of selected maternal and infant characteristics. In particular, the percentage of births to unmarried women and women receiving no prenatal care may be markers for behavioral risk factors (e.g., cocaine use), psychosocial risk factors (e.g., stress), and environmental risk factors (e.g., infection) for preterm delivery (6,9). In addition, race may be a marker for these factors. Risk markers may be useful for identifying groups at greatest risk for preterm delivery and for targeting prevention and education efforts. Moreover, race-specific variation in the rate of preterm-LBW infants may reflect differences in these behavioral, psychosocial, or environmental factors.

In the United States, race-specific differences in the incidences of LBW, particularly preterm-LBW infants, and infant mortality have increased (3,10). Further studies are needed to evaluate the relative importance of risk factors and to test strategies for prevention of preterm delivery (e.g., increasing access to comprehensive health care) in specific population subgroups.

References

1. Public Health Service. Healthy people 2000: national health promotion and disease prevention objectives -- full report, with commentary. Washington, DC: US Department of Health and Human Services, Public Health Service, 1991; DHHS publication no. (PHS)91-50212.

2. Taffel SM. Trends in low birth weight: United States, 1975- 1985. Hyattsville, Maryland: US Department of Health and Human Services, Public Health Service, CDC, NCHS, 1989. (Vital and health statistics; series 21, no. 48).

3. NCHS. Advance report of final natality statistics, 1991. Hyattsville, Maryland: US Department of Health and Human Services, Public Health Service, CDC, 1993. (Monthly vital statistics report; vol 42, no. 3, suppl).

4. CDC. Infant mortality -- United States, 1991. MMWR 1993;42:926- 30.

5. Kleinman JC, Kessel SS. Racial differences in low birth weight: trends and risk factors. N Engl J Med 1987;317:749-53.

6. Berkowitz GS, Papiernik E. Epidemiology of preterm birth. Epidemiol Rev 1993;15:414-43.

7. Hogue CJR, Buehler JW, Strauss LT, Smith JC. Overview of the National Infant Mortality Surveillance (NIMS) Project -- design, methods, results. Public Health Rep 1987;102:126-38.

8. Kramer MS. Determinants of low birth weight: methodological assessment and meta-analysis. Bull World Health Organ 1987;65:663- 737.

9. Rowley DL, Hogue CJR, Blackmore CA, et al. Preterm delivery among African American women: a research strategy. Am J Prev Med 1994;9(suppl):1-6. 

10. CDC. Differences in infant mortality between blacks and whites -- United States, 1980-1991. MMWR 1994;43:288-9.

TABLE 1. Rate * of low birthweight (LBW) +, by year, race of mother, and gestation & -- United States,
1981-1991
========================================================================================================
                                                             Year
                         ---------------------------------------------------------------------------
Race/Gestation            1981   1982   1983   1984   1985   1986   1987   1988   1989   1990   1991
----------------------------------------------------------------------------------------------------
White
  LBW, term               24.4   23.7   23.6   23.2   23.0   22.9   22.9   22.8   22.4   22.0   22.0
  LBW, preterm            31.0   31.3   31.7   31.2   31.9   32.0   32.3   32.0   34.4   34.7   35.7
  Total                   55.4   55.0   55.3   54.5   54.8   54.8   55.2   54.8   56.8   56.7   57.8

Black
  LBW, term               52.3   50.6   50.5   49.9   48.7   48.6   48.2   48.7   48.3   46.5   47.2
  LBW, preterm            72.1   72.7   74.6   72.8   74.2   75.5   77.4   79.2   85.7   85.3   87.7
  Total                  124.4  123.3  125.2  122.7  122.9  124.1  125.6  127.9  134.0  131.8  134.9

Overall @
  LBW, term               29.0   28.1   28.0   27.6   27.1   27.1   27.1   27.2   27.0   26.3   26.5
  LBW, preterm            37.5   37.7   38.4   37.8   38.4   38.9   39.6   39.6   42.9   43.0   44.3
  Total                   66.4   65.9   66.4   65.4   65.5   66.0   66.7   66.9   69.9   69.4   70.8
----------------------------------------------------------------------------------------------------
* Per 1000 live-born infants.
+ <2500 g (<5 lbs, 8 oz).
& Term is >=37 completed weeks of gestation; preterm is <37 completed weeks of gestation.
@ Comprises white, black, and other races.
========================================================================================================

TABLE 2. Percentage distribution of selected maternal and infant characteristics, by
race of mother -- United States, * 1981-1991 +
==========================================================================================
                       White            Black          Overall &
                   -------------    -------------    -------------
Characteristic     1981    1991     1981    1991     1981    1991
------------------------------------------------------------------
Age group (yrs)
    <18             4.2%    3.8%    11.6%   10.2%     5.3%    4.9%
  18-19             8.5%    7.2%    14.0%   12.8%     9.3%    8.0%
  20-24            33.3%   25.6%    35.4%   32.1%    33.3%   26.5%
  25-29            32.6%   30.9%    23.6%   23.9%    31.2%   29.7%
  30-34            16.7%   22.8%    11.4%   14.6%    16.1%   21.6%
  35-39             4.0%    8.4%     3.4%    5.5%     4.0%    8.1%
   >=40             0.7%    1.3%     0.7%    0.9%     0.7%    1.3%

Marital status
  Married          88.5%   78.3%    43.4%   32.1%    81.4%   70.6%
  Not married      11.5%   21.7%    56.6%   67.9%    18.6%   29.4%

Prenatal care
  First trimester  79.7%   79.6%    62.5%   62.0%    76.8%   76.4%
  Second trimester 16.2%   15.8%    28.7%   27.5%    18.3%   18.0%
  Third trimester   3.1%    3.3%     6.2%    6.1%     3.7%    3.8%
  None              0.9%    1.3%     2.6%    4.4%     1.2%    1.8%

Singleton birth
  Yes              98.1%   97.7%    97.5%   97.2%    98.0%   97.6%
  No                1.9%    2.3%     2.5%    2.8%     2.0%    2.4%
------------------------------------------------------------------
* New Mexico did not report date of last normal menstrual on birth certificates in 1981.
+ Percentages may not add to 100 because of rounding.
& Comprises white, black, and other races.
==========================================================================================

TABLE 3. Actual and standardized incidence * of low birthweight + (LBW) and change
in incidence, by race of mother and gestation & -- United States, 1981-1991
=======================================================================================================
                                   Actual                    Standardized @
                        ---------------------------  ----------------------------
                          Incidence      Absolute      Incidence       Absolute
                        ------------   change from   ------------    change from
Race/Gestation           1981   1991   1981 to 1991   1981   1991    1981 to 1991
---------------------------------------------------------------------------------
White
  LBW, term              24.4   22.0      -2.4        25.5   21.4        -4.1
  LBW, preterm           31.0   35.7       4.7        32.7   34.2         1.5
  Total                  55.4   57.8       2.4        58.1   55.5        -2.6

Black
  LBW, term              52.3   47.2      -5.1        54.0   45.8        -8.2
  LBW, preterm           72.1   87.7      15.6        75.2   84.2         9.0
  Total                 124.4  134.9      10.5       129.2  129.9         0.7

Overall **
  LBW, term              29.0   26.5      -2.5        30.6   25.4        -5.2
  LBW, preterm           37.5   44.3       6.8        40.1   41.8         1.7
  Total                  66.4   70.8       4.4        70.7   67.3        -3.4
---------------------------------------------------------------------------------
 * Per 1000 live-born infants.
 + <2500 g (<5 lbs, 8 oz).
 & Term is >=37 completed weeks of gestation; preterm is <37 weeks of gestation.
 @ Standardized incidences were calculated by direct standardization using the 1981 and 1991 combined
   population distributions of maternal age, marital status, and receipt of prenatal care, and
   infant's singleton status.
** Comprises white, black, and other races.
=======================================================================================================

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