Secular trends in the prevalence of low birthweight infants in Japan from 1980 to 2020: a joinpoint regression analysis

ABSTRACT Japan has a higher prevalence of low birthweight infants (LBW, < 2500 g) than other developed countries. In this study, we aimed to clarify the descriptive epidemiological characteristics of LBW prevalence in Japan from 1980 to 2020. LBW prevalence data were obtained from the Vital Statistics of Japan. Trends in crude and age-standardized LBW prevalence rates for total and singleton births separately and rates for full-term births, as well as age-specific prevalence rates by maternal age at delivery, were analyzed using joinpoint regression. The crude LBW rate increased slowly from 5.18 per 100 births in 1980, rose sharply from the late 1980s, peaked at 9.62 in 2007, and then declined slowly to 9.22 in 2020 for the total births. The annual percent changes were 1.35% (1980–1987), 3.37% (1987–2000), 1.81% (2000–2006), and –0.24% (2006–2020), with an average annual percent change (APC) of 1.51%. Age-standardized rates for the singleton and full-term births showed a similar trend. Age-specific rates showed a sharp increase for approximately 10 years after 1985, followed by a decline in the group > 35 years of age. The rate for the 15–19 age group increased gradually, with no change point (APC = 0.60). These findings likely reflect changes in maternal and socio-environmental factors—such as increased maternal age, undernutrition, lower body mass index (BMI), and expanded use of assisted reproductive technology and perinatal care—and emphasize the importance of preconception care and maternal health care, including nutrition and life-course approaches, to reduce low birthweight risk.