Declining Minamata male birth ratio associated with increased male fetal death due to heavy methylmercury pollution

The purpose of this retrospective study was to examine the effect of methyl mercury pollution on the sex ratio of offspring at birth and of fetuses at stillbirth in Minamata City, Japan, in the 1950s when severe and widespread methylmercury pollution was experienced. In 4 of 5 years from 1955 to 1959 when methylmercury pollution was most. severe, lower numbers of male offsp ring at birth were observed in the city population. The offspring sex distr ibutions from 1950 to 1969 by 5-year period were calculated in the overall population of Minamata City, in the most prevalent area, in fishermen (most heavily exposed occupation group) and among Minamata disease patients usin g data from birth certificates. We also similarly calculated the sex ratio of stillborn fetuses in the city population using Kumamoto Prefecture's vit al statistics on stillbirth. Decreases in male births were observed in offs pring in the overall city population, in fishermen, and in maternal Minamat a disease patients in the city in 1955-1959, when the methylmercury polluti on was most severe. An increase in the proportion of male stillborn fetuses in the city was observed at the time. It is possible that male fetuses wer e more susceptible to the pollution than their female counterparts, and thi s could be a cause for the lower numbers of male offspring at birth. (C) 20 01 Academic Press.