In the first half of your 28-day cycle, estrogen encourages the release of collagen, elastin, and hyaluronic acid thus improving your skin’s elasticity, hydration, and thickness. During this time your skin looks fresh and moisturized. When you enter the second half of the month, progesterone levels begin to increase causing the production of sebum. This prompts your pores to close up, often triggering hormonal acne. When estrogen and progesterone hormone levels fluctuate, testosterone which remains a constant during the cycle can cause a build-up of oil on your skin.