Various studies of persons exposed in utero were carried out during the 1940s and 1950s. Beginning in 1960, two overlapping fixed cohorts, one for clinical studies and one for mortality follow-up, were assembled from records of about 10,000 births occurring in or near Hiroshima and Nagasaki between the time of the bombings and the end of May 1946.

The clinical cohort included all Japanese survivors in utero within 1,500 meters of the hypocenters ATB, together with sex- and city-matched samples of similar size from two comparison groups. This cohort includes about 1,600 persons, of whom a subset of about 1,000 were enlisted as voluntary participants in the Adult Health Study (AHS).

The mortality cohort includes about 2,800 subjects, one fourth of whom are also in the clinical cohort. The mortality cohort was established in 1964 and consists of all persons exposed in utero within 2,000 meters of the hypocenters with matched comparison groups.

Current analyses of mortality and cancer incidence among persons exposed in utero make use of combined data from these two cohorts. This combined group now includes approximately 3,600 persons of whom 1,060 have estimated doses of 0.005 Gy or more (mean dose 0.28 Gy).