The Contribution of Maternal Birth Cohort to Term Small for Gestational Age in the United States 1989–2010: an Age, Period, and Cohort Analysis

Birthweight is highly correlated with neonatal and infant morbidity and mortality as well as later-life chronic disease, mental health, and socioeconomic status (SES) and is therefore often considered a measure of both current and future population-level health. Mean birthweights in the U.S. climbed steadily throughout much of the twentieth century.1, 2 In the late 1990s, however, mean birthweight began to decrease and rates of low birthweight (LBW) began to increase.3 Because infant birthweight reflects both adequacy of fetal growth and length of gestation, much of this secular decline was at first attributed to a shift in the distribution of gestational age due to increased use of procedures such as induction of labor and cesarean delivery.3 Declining birthweights were also attributed to demographic shifts, such as increasing proportions of multiple births and births to older mothers 3. However, a recent study showed that mean birthweight declined substantially between 1990 and 2005 at every week of term gestation (weeks 37–41).4 Moreover, the overall decline in birthweight was almost 80g over the 15-year period among a subgroup of singleton births to low-risk women (25–29 years of age, non-Hispanic white, ≥13 years of education, married, prenatal care in the first trimester, non-smoking, no pregnancy complications, vaginal delivery, no labor induction, prenatal ultrasound, and pregnancy weight gain of 26–35 pounds).4 These findings—which demonstrate a secular decline in birthweight in the U.S. that cannot not be explained by changes in length of gestation or traditional demographic, socioeconomic, medical care or behavioral risk factors—indicate a need for research into other, unexplored population-level determinants of fetal growth. In this vein, growing evidence suggests that maternal health and social factors prior to pregnancy, particularly during the in utero and early childhood periods, may influence later-life risk of adverse birth outcomes. Examples of such factors include childhood nutrition and growth,5–7 exposure to financial and social hardships,8 and being born in a poor neighborhood.9 Unfortunately, population-level data on maternal exposures early in life linked with later-life birth outcomes are scarce, especially in the U.S. Birth cohorts, however, can serve as proxy measures of both individual and contextual socioeconomic, behavioral, and environmental exposures over the life course.10 Cohort effects are often contrasted with age effects (which measure factors that change as individuals age) and period effects (which measure factors that change over chronological time). The objective of this study was to examine the contribution of maternal birth cohort (independent of maternal age and period of birth) to fetal growth over the last two decades in the U.S. Our central hypothesis is that mothers’ health and social exposures prior to pregnancy contribute to fetal growth of their own infants and that secular changes in these exposures may contribute to population-level trends in fetal growth. To test this hypothesis, we examined age, period, and cohort (APC) models of small for gestational age (SGA) overall and among term births only in the U.S. from 1989–2010. In light of stark racial/ethnic disparities in fetal growth11 and substantial racial/ethnic differences in cohort experiences in the US, we also hypothesized that maternal birth cohort associations with term SGA would differ by race/ethnicity. We therefore stratified our analysis by race/ethnicity and nativity, focusing on the four largest groups in the U.S.: foreign- and U.S.-born Hispanics, non-Hispanic blacks, and non-Hispanic whites.