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The ethynylestradiol concentration – in the presence of ethynodiol diacetate – in serum after oral administration was measured by a rapid radioimmunoassay method developed by the authors. It was found that the peak level was reached 1 h after administration, and even after 12 h a significant amount of free ethynylestradiol was present in the serum. The transfer of ethynylestradiol into the placenta was also studied in subjects who were 10–12 weeks pregnant. Placenta/serum quotients were calculated for the ethynylestradiol, and were found to increase in parallel with the dose of the drug administered, proving that an ethynylestradiol enrichment of the placenta occurred as early as 10–12 weeks of pregnancy.

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It was found by radioimmunoassay that the ethynyl estradiol concentration of the cervical mucus was 8.8 times higher than that of the serum in the first third of the cycle, 14 times higher in the second and 7.3 times higher in the third. The ethynyl estradiol content of the cervical mucus in the middle of the cycle showed a marked increase even when expressed as protein ratio. The accumulation of ethynyl estradiol in the cervical mucus is best demonstrated by the cervical mucus/serum ethynyl estradiol quotient of 9.7 found 12 h after oral administration of the contraceptive Bisecurin®.