Recent trend of increase in proportion of low birthweight infants in Japan.

BACKGROUND The proportion of low birthweight babies (LBW: 1500-2499 g) in Japan decreased steadily from 1950 to the 1970s. However, since then it has started to increase consistently, the reason for which has not been discussed in detail. METHODS Trends of birthweight and the two known factors for low birthweight (maternal smoking, and pre-pregnancy weight) were analysed with vital statistics, national nutritional and smoking prevalence survey data. RESULTS Increase in term LBW due to intrauterine growth retardation, is the major risk factor for the overall increase since the 1970s. The increase of smoking prevalence in women in their thirties started in the 1970s, while that for women in their twenties started in the 1960s. The decrease of body mass index for women in their thirties also began in the 1970s, while that for women in their twenties began in the 1960s. The ratio of delivery to mothers in their thirties to mothers in their twenties has increased more than threefold compared to the late 1970s. CONCLUSIONS Since the 1970s increase in smoking prevalence and decrease in body mass index in young women, especially those in their thirties, appeared to be the major factors involved in the increase in LBW babies.