The population of the Semipalatinsk region of Kazakhstan was chronically exposed to radioactive fallout from above-ground nuclear tests conducted during 1949–1956 by the Soviet Union. We investigated the effect of radiation exposure and other factors on risks of twinning overall and of same- and different-sex twinning and hence estimated dizygotic and monozygotic twinning rates in 11,605 deliveries around Semipalatinsk, 141 of which were twin, to 3992 mothers exposed to fallout during 1949–1956. Overall, the same-sex twinning rate was 7.85 [95% confidence interval (CI): 6.24, 9.47] per 1000 and the opposite-sex twinning rate was 4.45 (95% CI: 3.23, 5.67). Twinning rates did not differ significantly between radiation exposure categories, parental age at main radiation exposure, or year of birth. Different-sex, but not same-sex, twinning increased with maternal age (Ptrend  =  0.04) but not with other demographic factors and was increased soon after radiation exposure [OR  =  4.08 (95% CI: 1.11, 15.07)] for births occurring within 5 years compared with more than 20 years after exposure; this effect was similar in villages with low and high radiation exposure, however, so interpretation is uncertain.

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