Abstract
Using data from the Oxford Survey of Childhood Cancers this paper examines the effect of foetal age and number of films used on the subsequent risk of childhood cancer associated with prenatal x-rays. X-rays early in pregnancy were taken for different reasons and required more films when compared with those taken in late pregnancy. Therefore, Mantel-Haenszel techniques were used to estimate the independent effects of (a) exposure age and (b) number of films. Age at exposure had a clearly significant effect; x-rays taken in the first trimester of pregnancy were 2.69 times as effective as x-rays taken in the third trimester. First trimester exposures were often the result of maternal illnesses, so these maternal illnesses were then included among the controlling factors. When this was done the first trimester x-rays were 2.73 times as effective as later exposures. First trimester x-rays were most strongly associated with the cancers which were diagnosed between 4 and 5 years of age. Although the number of films had no detectable effect upon relative risk calculated over all ages, multiple exposures were demonstrably associated with early age at diagnosis.