Impetigo herpetiformis, also known as pustular psoriasis of pregnancy, is a rare pustular dermatosis typically seen in the third trimester of pregnancy. The exact prevalence is unknown, but there have been fewer than 200 reported cases to date.

It is unclear whether impetigo herpetiformis is a separate entity from generalized pustular psoriasis (GPP). Many women who develop impetigo herpetiformis have no personal or family history of psoriasis. Some women with biopsy-proven pustular psoriasis in pregnancy subsequently develop GPP later in life.

Pathogenesis is poorly understood, but it may be related to the increase in progesterone in late pregnancy. Impetigo herpetiformis can also arise during menstrual cycles and affect patients taking oral contraceptive pills, further supporting a role of hormones in pathogenesis. Some authors relate impetigo herpetiformis to the hypocalcemia that may develop in late pregnancy due to volume expansion and increased renal clearance, and hypocalcemia and low vitamin D can be seen in impetigo herpetiformis in the absence of pregnancy. Impetigo herpetiformis is also sometimes accompanied by hypoparathyroidism. Drug-induced cases have been reported in patients taking ritodrine hydrochloride, which has been used to suppress preterm uterine contractions.

The eruption usually begins in the third trimester of pregnancy (after 26 weeks) but may occur at any time during pregnancy. It commonly recurs in subsequent pregnancies, typically with earlier onset and greater severity with each successive pregnancy. It may be accompanied by systemic symptoms and signs including fever, chills, nausea, vomiting, diarrhea, malaise, and lymphadenopathy. Further maternal complications include electrolyte imbalance, fluid loss, and sepsis from secondary infection. Hypocalcemia can result in delirium, tetany, and convulsions. Fetal complications include placental insufficiency, intrauterine fetal demise, neonatal death, and low birth weight.