Stewart was one of the earliest to study the effect of prenatal X-rays, later replaced by ultrasound. She found that the children of mothers who received these X-rays were almost twice as likely to develop leukemia or cancer as other children. With J.W. Webb, B.D. Giles, and D. Hewitt. Stewart's follow-up paper was "A survey of childhood malignancies," British Medical Journal, 2 (1958) 1495-1508., with J.W. Webb and D. Hewitt.

In this study of ten million children Stewart and Kneale showed that obstetric X-rays significantly increased the rate of childhood leukemia and cancer.