DERMATOSES OF PREGNANCY:

There are a number of minor skin changes during pregnancy. There is an increase in spider naevi, melanocytic naevi, skin tags and chloasma. The abdomen shows midline pigmentation (linea nigra) and striae (stretch marks). There are four less common skin problems associated with pregnancy.
Polymorphic eruption of pregnancy (PEP)
This rash tends to appear in the last trimester of a first pregnancy in 1 in 160 cases. It is of unknown aetiology and recurs only rarely in subsequent pregnancies. It presents with very itchy urticated papules and plaques and occasionally small vesicles. Lesions usually start on the abdomen and striae but may spread to the upper arms and thighs. The umbilicus may be spared. PEP is commoner in twin pregnancies. The rash is not associated with any maternal or fetal risk. PEP has been shown to be associated with low maternal serum cortisol levels.
Treatment is with reassurance, bland emollients and mild topical steroids. The rash disappears after childbirth.
Prurigo of pregnancy
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This affects 1 in 300 pregnancies. It usually starts on the abdomen in the third trimester but may persist for some months after delivery. Clustered excoriated papules (prurigo-like lesions) occur on the abdomen and extensor surfaces of the limbs. The cause is unknown but pregnancy-related itch (pruritus gravidarum) may be due to cholestasis (p. 393). Rarely liver function tests are abnormal and urinary HCG levels may be elevated. It can recur in subsequent pregnancies. Some authors believe the condition is associated with an increase in fetal mortality but this remains controversial.
Treatment is with topical steroids and oral antihistamines.
Pruritic folliculitis of pregnancy
This occurs in the second or third trimester of pregnancy and is characterized by an itchy folliculitis which looks similar to steroid-induced 'acne'. It is not associated with any increased maternal or fetal risk.
Treatment with topical benzoyl peroxide and hydrocortisone cream help relieve symptoms.
Pemphigoid gestationis (herpes gestationis)
This is the rarest of the pregnancy-related rashes (1 in 60 000). The immune changes of pregnancy appear to set off bullous pemphigoid. It is characterized by an itchy blistering urticated eruption starting on the abdomen but may become widespread. Large bullae may be present. Unlike PEP it can occur early, starting in the second or even first trimester of pregnancy, and the umbilicus is often involved. It tends to recur in subsequent pregnancies and at an earlier stage. Diagnosis is confirmed by immunofluorescence studies.
A transient bullous eruption occurs in 5% of infants, presumably owing to transplacental passage of the offending antibody. There is no increase in fetal mortality but there is an increased incidence of prematurity and low birth weight, which is probably due to the autoantibody causing placental insufficiency. Therefore, it seems sensible to keep such pregnancies closely monitored and to advise on hospital rather than home delivery.
Treatment of mild cases may be with potent topical steroids but most cases will require oral corticosteroids. The steroid dose may need to be increased after delivery as there is often a postpartum flare-up of the disease. The rash can be set off again by the oral contraceptive pill and this should be avoided.

FURTHER READING
Kroumpouzos G et al. (2001) Dermatoses of pregnancy. Journal of the American Academy of Dermatology 45: 1-19.