Increase in low-birth-weight infants in Japan and associated risk factors, 1980-2000

Aim: To investigate possible factors related to the recent rise in prevalence of low-birth-weight (LBW) infants in Japan. Methods: A data set comprising 11 746 infants from the Children and Infant Growth Surveys (1980,1990, and 2000) was analyzed. Results: The proportion of LBW infants was 4.2% in 1980, 6.1% in 1990, and 8.3% in 2000. The maternal smoking prevalence increased from 6.5% in 1990 to 10.9% in 2000. When multivariate logistic regression analysis was applied to estimate the risk for LBW from 1990 to 2000, the following were selected as independent factors: preterm delivery, early term delivery, female sex of the infant, maternal primiparity multiple gestation, maternal short stature, older maternal age (>24 years), and maternal smoking. The population attributable fraction (PAF) of preterm plus early term delivery and multiple gestations to LBW was 85.1% in 1990, and 89.3% in 2000. The PAF of maternal smoking was 6.4% in 1990, and 7.4% in 2000. Conclusions: The increase in preterm deliveries and multiple gestations were found to be the important factors with regard to the increase in LBW infants in Japan. The increased prevalence of maternal smoking was not substantially associated with the increase in LBW infants.