Joyce the progestin in Mirena supresses the growth of the uterine lining and sometimes also ovulation. This is why many women have no cycles when it is fitted, though most, when you ask them to think closely about it for a cycle or two, can identify when their period would have been there from PMT-like periods followed by light cramping. I believe my lining continued to be thin after removal, causing my 2 pregnancies to fail to implant well enough to be sustained. 18 days on aspirin and RLT seems to have helped a whole lot.
Plan B is made of levonorgestrel. It disrupts the body's natural progesterone production by stimulating the body's progesterone receptors and telling it that there is plenty about - this makes the body think it has already ovulated and must not bleed. In Plan B this delays ovulation to prevent pregnancy. In other long-term contraceptives it effectively puts your body into the state it would be at in the middle of the luteal phase - it is sustaining the endometrial lining (butNOT building it, estrogen builds, progesterone sustains) and not ovulating perpetually until you have it removed. If you take it when you're pregnant it protects and supports the pregnancy. If you take it before you get pregnant it inhibits ovulation and the growth of the endometrial layer because it disrupts the natural oestrongenic period of the cycle when these things are usually happening.
This really makes sense to me because my second pregnancy i lost at around 7 weeks. I had spotting from 5 weeks, and it was very obvious to me that a) my hcg was extremely low throughout, and b) my body was doing it's damndest to hang on to the pregnancy despite very low hcg - to me this makes sense as i believe the still-high progesterone levels both made my lining inadequate for good implantation to occur AND helped me maintain the pregnancy despite this poor implantation. Given how the tissues of the pregnancy looked when i passed them i would say without the residual mirena progesterones i would probably have lost that pregnancy at 5 weeks, if i even made it that far. I also bled less during that miscarriage than i used to at a normal AF.
So it's not that i think progesterone causes miscarriage, but that i know that having too much of it in the first half of the cycle can have a disruptive effect on normal fertility mechanisms which can then lead to miscarriage. And it's funny you should mention twins because this current pregnancy is giving incredibly dark lines on the test sticks, much darker than i ever had with DD, and i am wondering how many are in there too!