As a rule, more boys than girls are born. But in November 1986 in the eastern regions of the Czech Republic, the reverse was true – more girls than boys. It appears that radiation exposure released by the Chernobyl nuclear accident in April 1986, brought to earth by rain over the area, increased radiation exposure. Fetuses that were approximately three months old at the time appear to have been effected, resulting in a reduction of newborn boys six months later.

According to the authors, “A greater vulnerability is known of male fetuses to prenatal damage by environmental stress. That is why the long-standing higher male birth fraction is considered as a sensitive indicator of stability and health in human reproduction. In this case, the authors hypothesize that “a large release of iodine-131 and other shorter-lived isotopes of iodine resulted in damage of the thyroid gland in fetuses and/or their mothers. A serious damage of the thyroid gland in three months old fetuses in end of April - beginning of May by radioiodine might be one of the main reasons for their loss detected as a decrease in male birth fraction in November 1986.”