Absorption of progesterone through the vagina is not transdermal - it is transvaginal - because as Joaniepat says - the lining is different and it is counted as skin.
The other things is that the aim of vaginal administration of progesterone is not absorption (through the vaginal mucosa - this is incidental) but delivery directly to the womb, I presume by diffusion ie it travels up the vagina through the cervix and directly to the uterus. This is the uterine first pass effect and is what makes it so effective. Progesterone from systemic absorption though the vaginal mucosa is transported in the blood stream and delivered indirectly in this way ( like transdermal oestrogen).
Also progesterone is a very unstable molecule so the formulations to deliver it to the body are more limited - so it can't be made into a patch for example. The biochemistry is complicated and beyond me - it can be delivered in gels, micronised in suspension, waxy capsules, creams etc but not as a patch due to its structure I presume.
If it were to be delivered transdermally as a cream then a formulation would need to be produced at a concentration that consistently delivers a given range of progesterone levels to women in order to protect the womb. Because progesterone is given chiefly either as a fertility aid or to protect the womb as part of HRT, then it is crucial to ensure any method of delivery actually guarantees to do this safely. The medical establishment can't put women at risk using a method that is possibly too variable?
Other hormones delivered through creams like testosterone (Androfeme) for example - it is given therapeutically so doesn't matter if we don't all get the same amount or if it's not absorbed properly.
What I don't understand is why, if progesterone can be produced in a gel ie Crinone (is this transparent??) - why a formulation can't be developed that can be absorbed through the skin using different carrier ingredients? I presume again that it might not be reliable enough or consistent enough to protect the womb?
Thinking aloud here Stellajane!
Hurdity x