We conducted a case-control study to investigate the relation between prenatal exposure to x-rays and childhood cancer, including leukemia, in over 32,000 twins born in Connecticut from 1930 to 1969. Twins as opposed to single births were chosen for study to reduce the likelihood of medical selection bias, since twins were often exposed to x-rays to diagnose the twin pregnancy or to determine fetal positioning before delivery and not because of medical conditions that may conceivably pre-dispose to cancer. Each of 31 incident cases of cancer, identified by linking the Connecticut twin and tumor registries, was matched with four twin controls according to sex, year of birth, and race. Records of hospitals, radiologists, and private physicians were searched for histories of x-ray exposure and other potentially important risk factors. Documented prenatal x-ray exposures were found for 39 per cent of the cases (12 of 31) and for 26 per cent of the controls (28 of 109). No other pregnancy, delivery, or maternal conditions were associated with cancer risk except low birth weight: 38 per cent of the cases as compared with 25 per cent of the controls weighed under 2.27 kg at birth. When birth weight was adjusted for, twins in whom leukemia or other childhood cancer developed were twice as likely to have been exposed to x-rays in utero as twins who were free of disease (relative risk, 2.4; 95 per cent confidence interval, 1.0 to 5.9). The results, though based on small numbers, provide further evidence that low-dose prenatal irradiation may increase the risk of childhood cancer.

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SUMMARY There is conflicting evidence about the effect of parental consanguinity on fetal growth. Previous studies have not always allowed for other factors that are known to affect birth weight, in particular, gestational age, parity, and maternal height. We have therefore studied this question in the Pakistani Moslem population in Birmingham. Babies born to parents who were first cousins were on average 80 g lighter than those born to unrelated parents, but this difference was not significant for the size of the sample studied. Nor were there any differences in the other measurements of the babies. After expressing birth weight in terms of centiles for gestational age, sex, parity, and maternal height, however, while there was no difference in the overall distribution of centiles, there were more poorly grown babies-that is, weight below the 10th centile-in the first cousin group. We conclude that parental consanguinity is associated with an increase in the number of poorly grown babies but that the overall effect on mean birth weight is small.While some studies have shown that parental consanguinity retards intrauterine growth,' 2 others have not found this effect.3 4 Even in the studies that found a reduction in birth weight, it was not apparent whether the difference in birth weight was explained by differences in gestational age, parity, maternal height, etc, and not all of the studies controlled for all of these factors when comparing birth weight from different types of marriages.In this hospital there have been increasing numbers of births to Pakistani mothers who are Moslems, followers of the Islam faith in which consanguinous marriages, particularly among first cousins, are preferred. In previous studies from this hospital it was suggested that environmental factors, such as the mother's health and nutrition in pregnancy, were more important in affecting intrauterine growth than genetic factors..In view of the conflicting evidence described above about the possible growth retarding effect of parental consanguinity, however, this study was designed to consider the effect of consanguinity on fetal growth after allowing for the major factors that affect intrauterine growth-for example, sex of baby, gestational age, maternal height, and parity. Patients and methodsBabies born to Pakistani mothers between November 1982 and May 1983 at our hospital were studied.The mothers were interviewed after the birth by an Asian interpreter who obtained details of the parental relationship, their place of birth, and obstetric history. Further maternal details were ascertained (by MH or LB) from the obstetric notes. The babies were examined within 48 hours of birth (by either MH or LB): any abnormalities were noted, gestational assessment made (Dubowitz method), and length measured by neonatometer, head circumference with a paper tape measure, and skinfold thickness with Harpenden calipers at the left triceps. The birth weight from the labour ward was recorded as measured by the attendant midwife on a spring balance and...