A woman’s skin may undergo significant physiological changes during pregnancy, including the presence of skin rashes. For many years, there was great confusion about which rashes were specific to pregnancy.

That changed with the development of immunofluorescence, a technique that uses antibodies (infection-fighting proteins made by the immune system) chemically linked to a fluorescent dye to identify antigens (substances that trigger an immune system response) in a tissue sample. Immunofluorescence has helped experts identify and distinguish the various forms of skin rashes associated with pregnancy.

Nonetheless, controversy remains regarding the classification of these rashes. To add to the confusion, experts routinely refer to each of these rashes by several different names.

Today, four major types of skin rashes are recognized as occurring only during pregnancy. One – pemphigoid gestationis – is clearly defined, while the others are so similar that they are sometimes grouped together. The characteristics of these rashes are:

• Pruritic urticarial papules and plaques of pregnancy (PUPPP). Also known as polymorphic eruption of pregnancy (PEP), it is the most common skin rash that occurs during pregnancy, affecting between one in 160 and one in 300 pregnant women. About 75 percent of cases occur in women who have not previously given birth.
  
  In addition, women who gain a lot of weight during pregnancy or who have greater stretching of their abdominal skin may be more susceptible to PUPPP. Some experts believe this is because stretching the skin may damage connective tissue, which exposes the antigens that trigger an inflammatory response. Others believe it is the result of the mother’s exposure to certain circulating fetal antigens. PUPPP also is eight to 12 times more common in women who are carrying multiple fetuses.
  

  

  PUPPP typically begins as small raised bumps (papules) within stretch marks on a woman’s abdomen. White rings may surround these papules.
  
  In many cases, the rash spreads to the extremities (arms and legs) and the individual papules fuse together to form hives. In most cases, the rash does not spread to the face, palms of the hands or soles of the feet. Most patients with PUPPP experience extreme itchiness, which may exacerbate other symptoms that occur late in pregnancy, such as sleep loss. This itchiness often worsens immediately after delivery and improves later.
  
  Women are most likely to experience PUPPP during the third trimester, although it also can occur earlier, or even after childbirth. It is unclear how likely women are to experience a recurrence of PUPPP in subsequent pregnancies. Menstruation and use of oral contraceptives usually do not trigger outbreaks. PUPPP occurs less commonly in Asians and African Americans than in Caucasians.
  
  
• Prurigo of pregnancy. Rash that has been linked to abnormal blood hormone levels, particularly elevated levels of gonadotropins and lower levels of cortisol and estrogen. This type of rash affects between one in 300 to one in 450 pregnant women. It usually appears in the second or third trimester and may cover the limbs and trunk of a woman’s body. Lesions tend to resemble insect bites and may become crusted over. In most cases, the rash resolves soon after delivery. However, prurigo of pregnancy may recur in subsequent pregnancies.
  
  Some experts believe that prurigo of pregnancy is overdiagnosed, and that many cases should actually be diagnosed as atopic dermatitis, a skin rash that occurs due to an allergic reaction.
  
  
• Pruritic folliculitis of pregnancy. A condition so similar to prurigo of pregnancy that some experts do not distinguish between the two. The incidence of pruritic folliculitis of pregnancy is unknown. The rash tends to appear in the second or third trimester and often resembles steroid-induced acne. It is most likely to affect the trunk of a woman’s body, although in some cases it may spread to the extremities. It usually resolves soon after delivery, and its likelihood of recurring in subsequent pregnancies is unknown.
  
  
• Pemphigoid gestationis. An uncommon blistering disease that occurs in one in every 1,700 to one in every 50,000 pregnant women. This type of rash usually erupts during a woman’s second or third trimester or even shortly after the child is born. In some cases, it may persist for years after a child has been born.
  
  Pemphigoid gestationis may begin as itchiness of the skin. When the rash erupts, it typically occurs on the trunk of a woman’s body, around the navel, and spreads quickly. The rash may affect the palms of the hand and soles of the feet, but usually does not appear on the face or mucous membranes (tissues that line body cavities).
  
  The rash itself is made up of different types of lesions, including raised dots or bumps and fluid-filled blisters. The rash may diminish before delivery, but returns following childbirth for many patients. Some women experience future flare-ups when using oral contraceptives or during menstruation. Women may also experience pemphigoid gestationis in subsequent pregnancies, although the condition may occasionally skip a pregnancy.
  
  Women who develop this type of rash are at increased risk of developing hyperthyroidism, a condition marked by excessive production of thyroid hormones. There may be an increased risk of premature birth or growth restriction for children whose mothers have pemphigoid gestationis. These children rarely are born with the condition themselves, but when it occurs, the rash is usually mild and goes away quickly.

A type of rash known as pustular psoriasis of pregnancy also sometimes occurs. However, it is very rare and experts are divided about whether or not it is truly specific to pregnancy. It can occur at any time during pregnancy and tends to appear first in skin folds (e.g., in areas around the groin, breasts, underarms, folds of knees). Eventually, it spreads to the trunk and extremities.

Patients with pustular psoriasis of pregnancy do not experience itching, but may have symptoms of illness such as malaise, loss of appetite, nausea, vomiting, diarrhea, fever and chills. Left untreated, this condition can have severe and even fatal health consequences for both the mother and the fetus.

Women who are pregnant also may experience common rashes that are not exclusive to pregnancy. For example, excessive dampness and perspiration may cause a heat rash to form. Other rashes that may occur include those caused by allergies, medications, insect bites, skin infections, viral infections, parasites and connective tissue disorders.

Rarely, pregnant women may experience severely itchy skin that is not accompanied by a rash. This is known as intrahepatic cholestasis of pregnancy (ICP) and it tends to occur during the third trimester. The condition interferes with liver function, causing bile acid levels to build up in the blood. Mild jaundice (yellowed skin and eyes) sometimes accompanies this condition. Women carrying multiple fetuses or who have a family history of ICP have a higher risk of developing the condition. Most often ICP is diagnosed after 30 weeks gestation, and it often recurs in subsequent pregnancies.

Aside from severe itching during pregnancy, ICP is not harmful for the woman, but it can be fatal for the fetus. The most common complication of ICP is premature birth, especially if ICP developed prior to 30 weeks gestation. The earlier in pregnancy a woman develops ICP, the greater the risk of premature birth. Stillbirth can also occur, most often during the last month of pregnancy. Physicians usually recommend close monitoring of a woman with ICP and encourage an early delivery, depending on how soon the lungs of the fetus are mature.