Transfer of polonium, neptunium, plutonium and americium to the primate fetus

Two baboons in the fifth month of pregnancy were given a single intravenous injection of a mixture of ²¹⁰Po, ²³⁷Np, ²³⁹Pu and ²⁴¹Am in citrate solution. Total retention in each fetus, measured at one week after injection, accounted for about 1% of administered ²¹⁰Po and ²³⁷Np, 4% of ²³⁹Pu and 0.4% of ²⁴¹Am. These values correspond to fetal:maternal whole-body concentration ratios, C(F):C(M), of about 0.3 for ²¹⁰Po, 0.6 for ²³⁷Np, 1.3 for ²³⁹Pu and 0.1 for ²⁴¹Am. Retention in the placenta varied from about 1% of administered ²³⁷Np to 8-12% of ²³⁹Pu. Available data for other species, including rats and guinea pigs, generally show lower levels of retention in the fetus and placenta, particularly for ²³⁹Pu. It is proposed that estimates of dose to the human fetus from intakes during the third trimester of pregnancy should be based directly on the baboon data.