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A Fever Blister in Pregnancy

By Heather Gloria ; Updated August 14, 2017

In the September 2009 edition of the “Journal of the Canadian Dental Association,” Dr. Giuseppe Ficarra and Dr. Catalina Birek explain that doctors and dentists don’t know how pregnancy influences the course and treatment of fever blisters. According to Ficarra and Birek, most women enter pregnancy with a history of exposure to the herpes simplex virus that causes fever blisters, although not all have a history of symptoms. A small number -- between 0.5 and 2 percent -- acquire the infection during pregnancy. These patients, say Ficarra and Birek, face greater risks of maternal and fetal complications.

Clinical Features

In the 2008 edition of “Archives of Internal Medicine,” Dr. Christina Cernik says that fever blisters present as painful 1- to 2-mm blisters filled with clear or cloudy yellow fluid that rupture spontaneously after three or four days. Many patients also experience a flu-like illness, particularly during the first episode. Pregnancy does not change the presentation of the fever blister. However, some pregnant women experience swelling of the mucous membranes of the mouth, particularly the gums, which may make it difficult to determine the extent of the fever blister and potentially delay healing.


Some patients experience more than one episode of fever blisters. Ficarra and Birek say health care providers don’t know how pregnancy influences a woman’s risk of fever blister recurrence. Anecdotally, they say, many women complain about an increased incidence of fever blisters during pregnancy. However, a 2003 study in the “International Journal of Gynecology and Obstetrics” found that the incidence of self-reported fever blister recurrences actually decreased slightly in early pregnancy compared to pre-pregnancy.


In rare cases, the herpes simplex virus can spread to eyes or brain. In fact, says herpes simplex infection of the eye is a leading cause of blindness in the United States. Similarly, the National Institute of Neurological Disorders and Strokes names herpes simplex as the most common cause of viral encephalitis, or infection of the brain, in the United States. Pregnant women with fever blisters can transmit the virus to their infants, resulting in death or disability, according to obstetrician Dr. F. Gary Cunningham in the 2008 edition of “Williams’ Obstetrics.”


Although fever blisters usually resolve without treatment, women who experience them during pregnancy should consult their obstetricians. The Food and Drug Administration designates acyclovir, valacyclovir and famciclovir, drugs commonly used to treat fever blisters, as “pregnancy category B” because they do not appear to cause birth defects, although adequate and well-controlled studies in humans have not been performed. A fourth drug, docosanol, is available without a prescription, but the FDA has not assessed its safety during pregnancy.


Women who have not previously been exposed to the herpes simplex virus should avoid new exposure during the pregnancy. Mayo Clinic women’s health nurse practitioner Lois McGuire says that women with partners who have a history of fever blisters should abstain from receiving oral sex during the third trimester of pregnancy, and the partner should consider taking antiviral drugs preventively. Women with partners who have tested positive for the herpes simplex virus with no history of lesions or with a history of genital lesions should abstain from all forms of intercourse during the third trimester of pregnancy.

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