Victor A., Lithell H., Selinus lngemar, and Vessby B. “Pharmacodynamics of a Contraceptive Vaginal Ring Releasing Norethindrone and Estradiol: Ovarian Function, Bleeding Control and Lipoprotein Patterns”. Upsala Journal of Medical Sciences, vol. 89, no. 2, June 1984, pp. 179-88, doi:10.3109/03009738409178478.