Prediction of fetal drug concentrations.

Equations are derived for the fetal concentrations of a drug [F(t)] during and after the slow release of the drug into the maternal circulation. To solve these equations it is necessary to know the placental drug clearance (Cp), the clearance of the drug from the fetal circulation by the fetal tissues (Cf), and the volume of distribution of the drug in the fetal compartment (V). With these values for acetylsalicylic acid (aspirin) it can be shown that the maximum fetal aspirin concentration will be much less than the maximum maternal concentration and that after the release of aspirin has stopped, some drug is trapped in the fetal circulation. Therefore, there will be a period of time when the fetal plasma aspirin concentration is greater than maternal plasma aspirin concentration.