Decline in sex ratio at birth after Kobe earthquake.

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.