Contributions of maternal lifecourse experience on temporal trends in birthweight in China: an age–period–cohort analysis among 2·5 million newborns

Abstract Background Small for gestational age (SGA), an indicator of small size at birth, can strongly predict mortality and morbidity in later life, and collective efforts to reduce the incidence of SGA or low birthweight can improve perinatal health. Previous studies have mainly focused on secular trends of birthweight by birth year, and few have explored the interaction between maternal lifecourse factors and trends in offspring birthweight. Rapid economic and social developments in China during past decades have profoundly affected women's health. In Guangzhou, a well developed city in China, rapid urbanisation along with an increased immigrant population provide a unique opportunity to explore the extent to which maternal lifecourse factors (age at delivery, period of delivery, and maternal birth cohort) independently contribute to trends in SGA incidence. Methods Data were obtained from the Guangzhou Perinatal Health Care and Delivery Surveillance System for 2 524 960 singleton livebirths with 24–42 weeks of gestation from 2001 to 2016. We used SGA as the birthweight indicator, defined as below the tenth percentile for sex-specific and gestational-age-specific birthweight standard from the INTERGROWTH-21st project. The age–period–cohort models were constructed to clarify the independent effects of age at delivery, period of delivery, and maternal birth cohort on SGA incidence trends. Analyses were also performed by stratifying maternal birthplace (western, central, and eastern China). This study was approved by the institutional ethical committee board of Guangzhou Women and Children's Medical Center. Data used in the study were anonymous, and individual identifiable information was not available for the analysis. Findings The SGA incidence declined from 10·4% in 2001 to 7·1% in 2016, with a 16-year overall incidence of 8·4%. In the age–period–cohort analysis, we observed strong effects of maternal age and birth cohort on SGA risk, while the effect of delivery period was mild. Age effect showed an L-shaped curve, with the highest risk (rates of >13%) for women younger than 20 years old irrespective of the delivery period. The effect of birth cohort on SGA incidence trends appeared to be linear. Compared with women born around 1961, those born around 1997 had a 61% decreased risk of SGA birth (95% CI 58–63). When the analyses were stratified by maternal birthplace, the effects of birth cohort seemed strongest among women who were born in central China compared with those from western and eastern China, whereas effects of maternal age and delivery period were similar among different birthplaces. Interpretation Our findings provide insight into the effects of maternal lifecourse factors on birthweight trends. The persistent effect of maternal age on risk of SGA births might represent the effect of biological factors, and the birth cohort effect implies that social transition during lifecourse (ie, poorer early life with substantial improvement later) plays an important part in perinatal health. Funding National Natural Science Foundation of China (81673181), Guangzhou Science and Technology Bureau, Guangzhou, China (201508020003, 201508030037), and Ministry of Science and Technology of the People's Republic of China (2016YFC1000205).