We investigated the possible association between the Kobe earthquake (January 1995) and the sex ratio among live-born infants after the catastrophe. A significant decline in the sex ratio (0.501) of Hyogo Prefecture in October 1995 was observed 9 months after the Kobe earthquake as compared with an expected value of 0.516 in the period from January 1993 to January 1996 (P = 0.04; one-tailed). Simultaneously, a reduction in fertility of approximately 6% was also observed, compared with the month of October 2 years previously. Thus, the acute stress resulting from a great natural catastrophe can be a cause of a low sex ratio at birth 9 months later.
PIP: This study examined the association between the Kobe earthquake in January 1995 and the sex ratio among infants born after the natural disaster in Japan. It is postulated that chronic exposure to environmental toxic agents of the male reproductive system could lead to low male/female ratios. Data were obtained from Hyogo Prefecture and published welfare statistics on the monthly number of males and females at birth during July 1995 and January 1996 compared to the same months in 1993. A conception during the earthquake would be a birth on October 9, 1995. Findings indicate a decline in the sex ratio in Hyogo Prefecture in October 1995. The proportion of males to females was 0.501 in October 1995 and 0.516 during January 1993 to January 1996. Differences were significant based on Fisher's 1-tailed test. Fertility declined about 6% between October 1995 and October 1993. The number of spontaneous abortions and preterm births remained the same between 1993 and 1995. The changes in sex ratio after the Kobe earthquake followed the same pattern of decline as the changes after the London smog and the Brisbane flood, but the decline appeared earlier. The sex ratio decline appeared in only about 280 days after the earthquake compared to 320 days in the other disasters. The authors speculate that the sex ratio changes may be due to acute stress and a reduced sperm motility. Evidence indicates sperm motility declined less than 1 month after the earthquake. Sperm motility was restored over a period of 2-9 months.