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How the Yuzpe method acts as an abortifacient

The mechanism(s) of action of the Yuzpe method of pre-implantation birth regulation have been the subject of research since the mid-1980s. A review of published papers indicates a consistent reference to a direct inhibitory effect on the proper development of the endometrium. As a consequence, implantation of the 5-6 day old human embryo is frequently blocked.

Illustrating this point was work by Kubba and co-workers (1986), who observed that the correct levels of natural, ovarian produced estrogen and progesterone are indispensable to the proper development of the endometrium to a stage that is compatible with blastocyst implantation. They further noted that the endometrium must be at the secretory phase of development for proper implantation, and theorised that a high dose of a progestin may disrupt this necessary development. Importantly, they named levonorgestrel as responsible for the retardation in endometrial growth.(1)

Supportive research was also reported by Rowlands and co-workers (1986), who found that the Yuzpe method works "at the endometrial level and operates before implantation." (2) More precisely, Rabone (1990) determined that 'Yuzpe' caused a 'desynchronisation' in the development of the endometrium, rendering implantation improbable. (3) Grou (1994), Harper (1995), the American Food and Drug Administration (1997), Trussell (1999), and Wellbery (2000) have all reported similarly. (4) (5) (6) (7) (8)

The American Food and Drug Administration's (FDA) product information for Preven™, a branded version of the Yuzpe method, lists three ways this product could work: delayed or prevented ovulation, changes to the viscosity of cervical mucus or it "may produce changes in the lining of the womb (uterus)." (9)

A research paper by Raymond et al (2000)(10) reported that the Yuzpe method had a statistically significant impact on 5 key characteristics of implantation, including a reduced endometrial thickness and an increase in estrogen receptors (the level should be decreasing for implantation), but no change in the levels of other uterine receptivity markers such as integrins or glycodelin when a control and treatment cycle in the same women was compared.

Raymond also reported an increase in oestrogen levels, which could have an anti-implantation action, because a decrease oestrogen level has been shown to improve uterine receptivity.

The authors also noted that "our results leave a puzzling gap in our understanding of the mechanism of action of this therapy ... in order for the Yuzpe regimen to be able to prevent 75% of expected pregnancies ... perhaps the (Yuzpe) regimen affects endometrial function in ways undetectable by the tests we chose to perform ..." (11)

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