The main hormonal trigger to acne in both men and women is testosterone. Progesterone inhibits 5α-reductase required to convert testosterone to the more potent Dihydroxy testosterone (DHT).
Progesterone that is out of sync with estrogen is usually the hormonal cause behind the dreaded pre-menstrual breakouts that many of you are so familiar with. It also plays a role in acne flare-ups that women get when they go off of hormonal birth control.
It seems that a low progesterone/estrogen ratio can increase insulin levels which in turn can lead to excess androgens, as well as amplify testosterone and DHT conversion in the skin. DHT is a potent acne-causing variant of testosterone that other forms of testosterone get converted into in the skin via an enzyme called 5-alpha reductase. Having a normal progesterone level in relation to estrogen can prevent 5-alpha-reductase from converting testosterone to DHT, which will, in turn, prevent acne.
Thus, it seems that progesterone in the right amount can decrease the severity of acne.