I think it's progesterone? I take it 3 times a day for 10 days, and it'll force a bleed when I go off it. And speaking from experience, I'll have about 3 weeks of humungous bleeding, changing maternity pads every 20 minutes and bleeding through clothes and onto furniture. And just as I start to feel faint and concerned, I'll go to the hospital - at which point I'll stop bleeding completely.

Been there, done that