Trends in the prevalence of low birth weight in Japan

Abstract

Low birth weight (LBW) is defined as a weight of 2499 g or less and in Japan it is considered an important determinant of infant health status. The number of LBW infants is included in monthly reports to the Ministry of Health and Labor and prefectural authorities. Reporting has been universal for all prefectures since 1973. The LBW rate in Japan declined until the 1970s, reaching a low point for the whole country in 1978–1979 when it was 5.2%, but has since steadily increased to 9.5%. The prefecture of Okinawa was returned to Japan in 1972 and the proportion of LBW declined from 8.1% in 1973 to a low of 7% in 1978, but then rose again to reach 10.9% in 2005. The LBW rate in Japan is now equal with Mexico, the highest of all the OECD countries. The total number of births in Japan declined from 1,246,802 in 1989 to 1,062,530 in 2005, but during this period the number of infants born with a birth weight below 2500 g increased from 75,527 to 101,272. Japanese mothers in the USA also have had an increase in the proportion of LBW births, but not nearly as great as in Japan itself. While increased rates of smoking, multiple births, and interventional obstetrics account for some of the increase in the LBW rate, most of the increase is due to the practice of restricting weight gain in pregnancy to a maximum of 8 kg. The neonatal mortality rate has continued to decrease despite the increase in the LBW rate, but at a cost of the provision of transport and neonatal intensive care. The high LBW rate will result in public health implications from the projected increase of chronic disease in adults. There is a need to change obstetric policy on weight gain during pregnancy to reverse the LBW rate trend.