When pustular psoriasis occurs in pregnancy it is sometimes referred to as impetigo herpetiformis. No one is sure if impetigo herpetiformis is simply pustular psoriasis in pregnancy, or a separate disease. A blood test may show lowered levels of calcium in patients with impetigo herpetiformis.
Like many names of dermatologic diseases, impetigo herpetiformis is actually a misnomer because pustular psoriasis is neither an infection (impetigo) nor related to herpesvirus (herpetiformis).
Impetigo herpetiformis often improves if an early delivery of the baby is possible, or with strong medications called corticosteroids. Although systemic corticosteroids are usually avoided for treatment of psoriasis, impetigo herpetiformis is the exception to the rule - in addition, the steroids given to the mother can help mature the infant's lungs allowing for an early delivery with possible cure of the skin eruption.
source: James WE, Timoth GB, Elston DM. Andrew's Diseases of the Skin. Saunders (Elsevier) Canada 2006 page 195