Why does acne increase during pregnancy?

Studies show a variable effect of acne in pregnancy. If treatment regimens that were working prior to pregnancy were abandoned, the patient can have initial flares that are not directly related to the pregnancy. Some studies show that as many as a third of cases actually improve in pregnancy, but most women will report some worsening. Progesterone, which has some androgenic components, is increased during pregnancy, resulting in more secretions from the skin glands.

Postpartum, some women will get acne for the first time (called postgestational acne). Maintaining hydration should help. Women should consult their doctor if a topical medication is needed. Azelaic acid, topical erythromycin or clindamycin, and oral erythromycin are all safe. Although topical tretinoin has not been reported to cause risk, no studies have established it's safety and it should be avoided. Tetracyclines are contraindicated during pregnancy. Appropriate cleansing with mild abrasion aids has been found to be helpful.

Source: http://emedicine.medscape.com/