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Factors that Appear to Determine the Extent of Placental Transfer of Drugs Administered to the Mother [1-5] The molecular weight of the drug; Drugs with molecular weights (MW)  less than 500 D appear to cross the placenta easily, drugs with MW greater than 500  have  incomplete transfer across the human placenta. Drugs with very high MW , such as insulin with a M.W ~ 5800 do not normally cross the placenta. The pKa (pH at which the drug is 50% ionised); Strongly dissociated acid drug molecules should have an incomplete transfer, but this does not seem to be an absolute rule Physiological properties of the placenta (blood flow, gestational age) Expression and activity of metabolizing enzymes and efflux transporters (e.g. ATP-binding cassette (ABC) transporters ). For example placental 11beta-hydroxysteroid dehydrogenase 2 (11beta-HSD2) enzyme inactivates prednisolone by interconversion into prednisone , and ABC transporters pump tenofovir disoproxil fumarate  from trophoblast back to maternal circulation. The extent of drug binding to plasma protein does not appear to influence the type of drug transfer across the human placenta to a significant degree.