Pustular psoriasis of pregnancy, also known as impetigo herpetiformis, 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 and are never symptomatic outside of pregnancy. However, some women with biopsy-proven pustular psoriasis in pregnancy subsequently develop GPP later in life.

Pathogenesis is poorly understood given the limited number of cases, 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 hypocalcemia in late pregnancy due to volume expansion and increased renal clearance.

The eruption usually begins in the third trimester of pregnancy (after 26 weeks) but may occur at any time during pregnancy. It begins as erythematous plaques with circumferential rings of pustules. Plaques can become centrally eroded as they enlarge, and patients may experience an itching or burning sensation, especially at the margins. Plaques often arise first in intertriginous areas and expand to the trunk and extremities symmetrically. The head, hands, and feet are usually spared. Pustular psoriasis of pregnancy may affect oral and esophageal mucous membranes, and subungual pustules can result in onycholysis.

The rash may be accompanied by systemic symptoms including fever, chills, nausea, vomiting, lymphadenopathy, seizure, and malaise. It commonly recurs in subsequent pregnancies, typically with earlier onset and greater severity with each successive pregnancy. 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.