Small-for-gestational-age births in the United States: an age-period-cohort analysis.
BACKGROUND
During the last 2 decades, the rate of low birthweight has increased, as has the rate of preterm delivery, among both whites and blacks. Examination of causes for these secular trends has focused largely on changes in the distributions of maternal age and, less commonly, on birth cohort. Little is known as to how age, period, and birth cohort interact on trends in small births at term.


METHODS
The U.S. natality files were used to assess trends in term (>/=37 weeks gestation) small-for-gestational age (SGA) births for 7 5-year maternal age groups (15-19 through 45-49 years), 6 delivery periods (1975, 1980, 1985, 1990, 1995, and 2000), and 12 5-year maternal birth cohorts (1926-1930 through 1981-1985). SGA births were defined as sex-specific birthweight below the 10th percentile for gestational age based on 1995 livebirths in the United States. Logistic regression models were fit to determine the independent effects of age, delivery period, and birth cohort on term SGA trends, separately for blacks and whites.


RESULTS
Between 1975 and 2000, term SGA births declined by 23% (from 21% to 16%) among blacks and by 27% (from 12% to 9%) among whites. Term SGA births declined with increasing age up to 30-34 years, but increased among older women. Within strata of maternal age, the risk also declined with later maternal birth cohorts, among both blacks and whites. The strongest influence on SGA trends was from maternal age, followed by maternal birth cohort, and lastly by delivery period. In general, for any combination of age, period, and birth cohort, blacks showed 1.5- to 2-fold higher rates of term SGA than whites.


CONCLUSIONS
The persistence of strong maternal age effects on risk of term SGA births suggests that the effect of age is at least partly the result of biologic factors. Term SGA trends were generally consistent for blacks and whites, although the magnitude of difference in the risks for combinations of age, period, or mother's birth cohort was higher among blacks than whites.
Ananth
Cande V
CV
Section of Epidemiology and Biostatistics, Department of Obstetrics, Gynecology and Reproductive Sciences, University of Medicine and Dentistry of New Jersey, Robert Wood Johnson Medical School, New Jersey 08901-1977, USA. cande.ananth@UMDNJ.EDU
Balasubramanian
Bijal
B
Demissie
Kitaw
K
Kinzler
Wendy L
WL
D000293:Adolescent; D000328:Adult; D044383:Black People; D015331:Cohort Studies; D005260:Female; D006801:Humans; D007230:Infant, Low Birth Weight; D007231:Infant, Newborn; D008423:Maternal Age; D008875:Middle Aged; D011247:Pregnancy; D012307:Risk Factors; D014481:United States / Q000453:epidemiology; D044465:White People; D001741:Black or African American