Twinning in southern Israel. Seasonal variation and effects of ethnicity, maternal age and parity.

Seasonality of births in southern Israel was examined in two populations, Jews and Bedouins, with distinctly different life-styles. The study included 1,444 twin births that occurred between the years 1970 and 1986. The peak month for the birth of monozygotic twins was September in both populations, while the maximum number of deliveries occurred in January for the Bedouins and August for the Jews. Of Jewish dizygotic twin births, higher rates were found from July to December. In addition, the peak months of singleton births in women aged 35 years and older and in women of high parity did not coincide with the peak months of multiple births. Maternal age and parity have been shown to greatly influence twin birth rates. The autumnal peak we found, which was independent of ethnic origin and its associated cultural and sociologic differences, was also independent of maternal age and parity and was consistent with findings in other populations in the northern hemisphere. That finding suggests that a seasonal factor, as yet undefined, affects the rates of multiple births.