How levonorgestrel-only acts as an abortifacient

The medical literature makes it clear that levonorgestrel has a high pre-implantation/emergency abortifacient action.

Work by Kubba et al (1986) specifically referred to levonorgestrel, noting its ability to change "the nature" of the hormonal receptors within the endometrium. (1) Dr Rabone (1990) reported that levonorgestrel caused a reduction in the number of estrogen and progesterone receptors within the endometrium. As Dr Rabone reported: "The concentration of these receptors is critical for the normal development of the endometrium to a stage that will support implantation." (2)

More specifically, the manufacturers of Plan B™, a proprietary brand containing 0.75mg of levonorgestrel, have stated that:

"Oral levonorgestrel acts as an emergency contraceptive principally by preventing ovulation or fertilization (by altering tubal transport of sperm and/or ova). In addition, it may inhibit implantation (by altering the endometrium). It is not effective once the process of implantation has begun (Prod Info Plan B™, 1999)" (3)

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(1) Kubba, op.cit., p.515

(2) Rabone, op.cit., p.46

(3) Micromedex Vol.106 Levonorgestrel monograph