The male/female ratio of fetal deaths and births in Japan

Since the 1950s, the ratio of male-to-female births has been declining in several industrial countries. In Japan, the proportion of male births decreased from 0.517 in 1970 to 0.513 in 1996. Although many factors have been identified as possibly affecting the male/female ratio of births, it is noted that the change in the male/female ratio of deaths in the country may have led to the change in the male/female ratio of births. Although fetal deaths have been continuously decreasing during 1959-1996, the male/female ratio of fetal deaths (after 12 weeks of gestation) was reported to have been increasing since the 1970s, reaching over 2.0 in 1996. This could explain the decreasing male/female ratio of births. This trend could suggest some circumstantial change which might affect fetal survival and increase the prenatal vulnerability of the male fetus. Other contributing factors include exposure of endocrine disruptors or other environmental toxins at the critical stage of sex differentiation and induced ovulation.