A randomized controlled double-blind study of the effects on hemostasis of two progestogen-only pills containing 75 microgram desogestrel or 30 microgram levonorgestrel - Winkler, U H; Howie, H; Bühler, K; Korver, T; Geurts, T B; Coelingh Bennink, H J (Contraception, 1998)