I've read a lot of thing about every kind of progesterones and progestins used in HRT, but cannot be sure if they are doing what they claimed to do, because I don't know people, who used them, but I can speak from my own experience:
I am 33 years old, on 13 months of HRT. My breasts started growing on the 3rd week on hormones and were growing until the 5th month. Then they just stopped and that was all. After I reached 1 year of HRT, I was still unsatisfied with the feminization process - the breast, the hair, the body hair, the fat distribution, curves and so on, so I decided to start Utrogestan, which is the same as Prometrium, just a different brand.
I am taking 200 mg. per day, in divided doses - 100 at morning and 100 at evening, sublingually, right after meal. So far I am taking it for 2-3 weeks and believe it or not, I am already seeing results - my hair changed in texture - now it is more female-like, my face looks somehow softer and rounder, my body hair reduced a little bit... I see nothing about my breasts, but I am sure they will start to grow soon, because the last days I feel unusual tenderness, similar to how I felt in the beginning of HRT, when they were about to grow. I even started to lose weight, which was impossible so far, during all my HRT, because I was gaining all the time, no matter how hard I exercised and being on diets...
Also, I feel much calmer and sleep better

No side effects so far