Progesterone rises in the second half of a woman's cycle to prepare for pregnancy (implantation of fetus and other reasons) as an ovum has just been released for fecundation. That's the only reason why it increases. Nature intended it to be that way as naturally, a woman would most likely become pregnant around that time. Progesterone would then continue to rise just as estrogen would until she gives birth after which prolactin levels (and oxytocin, I think which helps solidify bond between infant and mother) remain high until breastfeeding ceases. Then, estrogen rises again, ovulation, and then progesterone rises to prepare for pregnancy again. In my opinion and I'm not a doctor, just an interested reader of the subject, this is the natural cycle of human females, not the highs and low of progesterone and estrogen every month which according to some, could be the contributing factor to breast cancer but this is not fact, just speculation, at this point.
Progesterone taken orally goes through extensive metabolism but how much varies according from one individual to another, depending on age, metabolism, health, genetics, other drugs/supplements, foods, beverages taken concomitantly. The advantage of taking it orally is its sedative effects as there is a higher level of its sedative metabolite orally than any other route. It will help if you are anxious to begin with, have problems with sleep. I personally like it this way.
Progesterone can be taken rectally (Cyclogest suppositories), capsules like Utrogestan, Prometrium can be even taken vaginally/rectally as genetic women have reported its effectiveness that way and doctors do allow for them to be taken that way in certain circumstances. It can also be taken in pellets or by injection but the latter may be inconvenient as injections have to be done every 1-2 days because levels drop quite rapidly. All these routes allow for much less metabolization and higher levels of P at lower doses, except perhaps for rectal where there still could be some P going through liver depending on where the suppository is placed (how far up).
There are alternatives to bio-identical progesterone, still apparently quite safe and free of androgenic effects. Dydrogesterone (Duphaston) oral tablets, 17-alpha hydroxyprogesterone caproate (Proluton Depot) injectables and even dienogest found in some pills with Estradiol Valerate.
Discuss those with your doctor, and hopefully you will find what is most suitable and effective for you.
