CONICET | Buscador de Institutos y Recursos Humanos

UPA is a selectiveprogesterone receptor modulator introduced into the market as the mosteffective emergency contraceptive pill in several countries. However, themechanisms underlying its action are not completely understood. Its primarymode of action is to inhibit or delay ovulation only if taken before LH peak.Considering its high effectiveness and previous results in mice showing it doesaffect neither gamete transport or interaction nor early in vitro embryo development, it is possible that this drug preventspregnancy by another mechanism. Based on this, the aim of this work was toevaluate potential effects of UPA on post-fertilization events in mice. First,females were caged with male breeders and successful mating was confirmed bythe presence of copulatory plugs (embryonic day 0.5:E0.5). At E1.5, a group ofmated females received 40 mg/kg UPA (i.p., previously shown to preventovulation) whereas a control group received vehicle alone (sesame oil). AtE14.5, the number of implantation sites and corpora lutea was recorded.Although pregnancy rates were similar in both groups, half of the UPA-treatedfemales presented lower number of implantation sites (UPA: 5.6±0.2 vs control: 11.9±0.5, p<0.05) despite similar number of corporalutea (10.8±1.1 vs 12.4±0.8, NS). The percentage of viable embryos was similarin both treatments. When the procedure was repeated but injecting UPA orvehicle closer to the window of implantation (E2.5), UPA-treated femalespresented significantly lower pregnancy rates (40% vs 100%, p<0.05) andnumber of implantation sites (1.2±0.8 vs 8±2, p<0.05). To evaluate an effect of UPA on embryotransport, embryos were collected by flushing the oviducts and uteri at E3.5.In both cases, all the embryos were recovered only from the uteri and mainly atblastocyst stage. Altogether, these results suggest that administration of UPAafter fertilization prevents pregnancy without affecting embryo transport ordevelopment.