I've not known a single case where transdermally applied progesterone was necessary to maintain a pregnancy, but am willing to learn. Questions arisen by that proposition include: 1) how much progesterone ? 2) which transdermal delivery formulation to use (dwell times and absorption rates vary), 3) which location(s) at which to apply it ? 4) when to apply it ? 5) when NOT to apply it ?
etc.
Progesterone peaks at about week 3 and then diminishes. Should the transdermal application vary in its administration as does the normal body level over those three weeks ?
I'd forget about transdermal anything, except avoiding the crap that is in shampoos, skin "conditioners", soaps, etc. because those fifty chemicals in your shampoo DO cross the skin.
I do note that there about six billion people on the planet, who came into the world without transdermal progesterone.