• sex ratio
• seasonality
• trend

Seasonality of sex ratio of live births (SR: male births divided by total births) has been reported in Europe, North America, Brazil, and Australia. However, no uniform pattern is seen.¹ Moreover, the magnitude of any observed seasonal variation varies from population to population with marked variation in Japan² to minor variation in Germany³ to none in south western Finland, Scotland, Costa Rica, and Hausa, Africa. The population of Novosibirsk region was 2 767 938 in 1988. Siberian climate exhibits considerable seasonal temperature changes. In Novosibirsk over the period 1951–1980, the average difference in mean monthly air temperature between January (the coldest month, −18.8°C) and July (the warmest month, 19.0°C) was 37.8°C. We tested the null hypothesis that there is no seasonal variation in SR in Siberia.

Records of live singleton births were obtained from the Novosibirsk Regional Committee for Statistics. Data by month were obtained for the years 1959–2001, excluding 1961, 1962, and 1988 because of missing data. Seasonal analysis was carried out by Edwards’ method. Our analysis was quarterly because of the comparatively small number of births. Linear regression analysis was performed to test for secular trend.

A highly significant seasonal pattern was evident (χ²=14.4, p=0.001) with an amplitude of 1.2 % of the overall mean, a peak in the second quarter (θ=129°) and a trough in the fourth quarter (fig 1).

A negative annual secular trend was found for the period 1971–1980 (r=−0.84, p=0.002), which was replaced by the positive trend during the period 1982–1993 (r=0.78, p=0.004). No difference in mean SRs for the entire period was found between urban (0.513) and rural (0.513) populations.

The decrease in male births in the last quarter equates to fewer male conceptions nine months previously—that is, in the first quarter. Climatic variations in west Siberia are extreme, with heavy snowfalls in winter. Thawing of snow requires considerable energy, therefore temperatures remain low in spring, and rise sharply from the second half of April. If the observed variation in SR is indeed temperature related, then it would seem that low temperatures either reduce male conceptions or, through unknown mechanism/s, reduce the survival of male conceptuses.

Industrialisation has been blamed for declining SRs in industrialised countries over the past half century. In Siberia, a different pattern is evident in that SR fell and then rose with a turning point in the early 1980s.

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Figure 1

Seasonality of sex ratio at birth in Novosibirsk region, Russia, 1959–2001. SR: male births divided by total births. Values are means and 95% confidence intervals.