The concentration of BPA in the fetus peaked within one hour after administration, reaching 11 ppb. This paper suggests that BPA passes easily through the placental barrier, bypassing protections that limit the impact of native estradiol and phytoestrogens. Arnold, SF, MK Robinson, A Notides, LJ Guillete, Jr. and JA McLachlan. A yeast estrogen screen for examining the relative exposures of cells to natural and xenoestrogens. . The somewhat forbidding title of this article may make one of its key results less than obvious.
Natural estrogens bind very tightly.
Contaminants less so, sometimes less by a factor of 1000x or more. This might be taken to indicate that they represent little risk. What this paper points out is that binding affinity is only one of a series of factors that control risk. Blood serum contains binding proteins which, in effect, reduce the amount of natural estrogen that is freely available to enter cells and interact with estrogen receptors. Arnold, SF, BM Collins, MK Robinson, LJ Guillette, Jr and JA McLachlan.