Secular trends of low birth weight, preterm birth, and small for gestational age in Shanghai from 2004 to 2020: an age-period-cohort analysis.

Background: Although highly heterogeneous among countries, the incidence rates of low birth weight (LBW), preterm birth (PTB), and small for gestational age (SGA) have been increasing globally over the past two decades. To better understand the cause of these secular trends, this study aimed to investigate the effects of age, period, and birth cohort on LBW, PTB, and SGA rates in Shanghai.

Methods: Data from 2,958,695 singleton live births at 24-41 gestational weeks between 2004 and 2020 were obtained for this study. Age-period-cohort models based on Poisson regression were used to evaluate the independent effects of maternal age, delivery period, and maternal birth cohort on the trends in LBW, PTB, and SGA.

Results: The overall prevalence rates of LBW, PTB, and SGA were 2.9%, 4.7%, and 9.3%, respectively, and significant changes were observed (average annual change: + 10.7‰, + 9.1‰, -11.9‰) from 2004 to 2020. Cohort effect increased steadily, from 1960 (risk ratio [RR] = 0.71, 95% confidence interval [CI]: 0.65-0.78) to 1993 (RR = 0.97, 95% CI: 0.94-1.01) for LBW and from 1960 (RR = 0.69, 95% CI: 0.64-0.75) to 2004 (RR = 1.02, 95% CI: 0.94-1.12) for PTB. A strong cohort effect was found with the highest risk of SGA (RR = 1.82, 95% CI: 1.72-1.93) in 1960 and the lowest risk (RR = 0.57, 95% CI: 0.54-0.61) in 2004, compared with the reference cohort of 1985. There was a "U-shaped" maternal age effect on LBW and PTB and a weak period effect on the three birth outcomes.

Conclusions: Our findings suggested a significant independent effect of age, period, and birth cohort on the three birth outcomes. The increasing rates of LBW and PTB motivated us to focus on young and advanced pregnant women. Meanwhile, the prevalence of SGA decreased steadily, illustrating the need for further research on the mechanisms underlying these trends.