Victor , A., Lithell , H., Selinus , lngemar and Vessby , B. (1984) “Pharmacodynamics of a Contraceptive Vaginal Ring Releasing Norethindrone and Estradiol: Ovarian Function, Bleeding Control and Lipoprotein Patterns”, Upsala Journal of Medical Sciences, 89(2), pp. 179-188. doi: 10.3109/03009738409178478.