OBJECTIVE: To evaluate whether climate change is associated with male:female ratios (sex ratios) of fetal deaths and births in Japan.

DESIGN: A population-based cohort study.

SETTING: Not applicable.

PATIENT(S): Newborn infants and fetuses spontaneously aborted after 12 weeks of gestation.

INTERVENTION(S): None.

MAIN OUTCOME MEASURE(S): Yearly sex ratios of fetal deaths and newborn infants and monthly fetal death rates and sex ratios of newborn infants.

RESULT(S): A statistically significant positive association was found between yearly temperature differences and sex ratios of fetal deaths; a statistically significant negative association was found between temperature differences and sex ratios of newborn infants from 1968 to 2012, and between sex ratios of births and of fetal deaths. The sex ratios of fetal deaths have been increasing steadily along with temperature differences, whereas the sex ratios of newborn infants have been decreasing since the 1970s. Two climate extremes, a very hot summer in 2010 and a very cold winter in January 2011, showed not only statistically significant declines in sex ratios of newborn infants 9 months later in June 2011 and October 2011 but also statistically significant increases of fetal death rates immediately, in September 2010 and January 2011.

CONCLUSION(S): The recent temperature fluctuations in Japan seem to be linked to a lower male:female sex ratio of newborn infants, partly via increased male fetal deaths. Male concepti seem to be especially vulnerable to external stress factors, including climate changes.