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

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After decades of steady increase, mean birthweight in the US declined throughout the 1990s and early 2000s, a trend not fully explained by changes in length of gestation, medical practice, demographics, or maternal behaviours. We hypothesised that secular changes in health or social factors across women's life courses may have contributed to this unexplained trend and examined maternal birth cohort as a proxy measure of life-course determinants of fetal growth in the US.


We used the age, period, and cohort (APC) intrinsic estimator (IE) approach to estimate the contribution of maternal birth cohort (independent of maternal age and period of birth) to small for gestational age (SGA), overall and among term births, in the US from 1989 to 2010. We conducted analyses separately among foreign- and US-born Hispanic, non-Hispanic black (NHB), and non-Hispanic white mothers.


We found evidence of a U-shaped relationship between maternal birth cohort and SGA among NHB women only. After accounting for maternal age and period of birth, risk of SGA among NHB women born in 1950 was 21.1% and decreased to 15.9% in 1970. However, NHB women born after 1970 experienced increasing risk (19.6% by the 1986 birth cohort).


Our findings suggest that NHB women born after 1970 have experienced increasing risk of SGA. Declining risk of SGA across NHB maternal birth cohorts from 1950 to 1970, however, suggests the potential to reverse this trend. Results illustrate the need for research on health and social risk factors for SGA across the pre-pregnancy life course.

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