The estimation of the absorbed radiation dose to the human fetus from59 Fe administered to the mother requires information on the biokinetics of the59 Fe within the mother and fetus. For the first part of pregnancy, up to about 24 wk gestation, information from studies done on women receiving therapeutic abortions is available. However, for the late gestational stages it is necessary to utilize data from animal studies. Maternal-fetal biokinetic studies were done on six late gestational stage ewes with chronic in-dwelling catheters in both the ewe and in utero fetus. The iron tracer,59 Fe, was injected into the maternal circulation simultaneously with an59 Fe injection into the fetal circulation. Transport between ewe and fetus was measured up to 110 hr after injection and tissue distribution was determined from autopsy specimens obtained at sacrifice. Fractional transport of the59 Fe tracer to the fetus was 0.64% of the dose given the ewe. Extrapolation of the sheep data to the human situation was accomplished by comparing early gestational stage human and sheep data. With these extrapolated data, the absorbed radiation dose to the 36-38-wk gestational age human fetus was estimated to be 25 rad per mCi of59 Fe administered intravenously to the mother.

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