Effects of levonorgestrel on capacity of mouse oocytes for fertilization and development

This study utilized a mouse model to examine whether levonorgestrel implants, such as Norplant, may exert their contraceptive effects in part by adversely affecting oocyte quality. Time-release pellets and silastic capsules containing levonorgestrel were implanted subdermally in mice and left in situ for periods of two or three months. After that time, mice were given injections of gonadotropins to stimulate ovulation and oocytes were obtained from the oviducts of the animals, inseminated, and subsequently cultured for 96 hrs, some in medium supplemented with levonorgestrel. The numbers of oocytes that were fertilized and the numbers that developed at least to the morula stage were determined. Levonorgestrel was not found to have an adverse effect on either fertilization or preimplantation development but instead was found in some cases to increase the proportion of oocytes fertilized and the proportion of embryos that developed to morulae. Results indicate that the contraceptive effect of levonorgestrel implants should be attributed to effects other than diminished oocyte quality and they further substantiate the safety of levonorgestrel, suggesting that it will not adversely affect early embryonic development in the event that a pregnancy should be initiated during its use.