Women who suffer from allergies may find that they get better, stay the same, or feel even worse in pregnancy — each individual's response is different. Symptoms are not life-threatening but can interfere with a good night's sleep and leave you feeling miserable. If you know what you're allergic to and are pregnant, try to avoid that thing (or things) rather than taking lots of medications. It also helps to decrease exposure to cigarette smoke and other known environmental irritants. If this isn't practical, however, keep in mind that some medications are safer to use than others while you are expecting.

It's not unusual for women who did not previously have allergies to suffer from nasal congestion while pregnant, as the hormones of pregnancy can cause swelling of the nasal passages and increased mucus production. In these cases, symptoms typically are worst in the second trimester and resolve within a few days after childbirth.

Seeking Relief
In general, it's best to avoid exposing a fetus to medications, especially in the first trimester when its organs are forming, but sometimes drugs are necessary either for medical reasons or to provide relief from symptoms. (For more information, see the article Medications in Pregnancy: General Principles, which includes how to understand the FDA safety classes for medicines.)

Allergy Prevention Medicines: Mast Cell Stabilizers
Cromolyn sodium helps prevent allergic reactions. It is available by prescription in eyedrops, nasal sprays, and inhalers. It falls into FDA category B and is considered safe in pregnancy. The brand name for these products includes Intal inhalers, Nasalcrom nose spray, and Crolom eyedrops.

Allergy Prevention and Treatment

  • Steroids
    Inhalant steroids are often used to treat asthma, and nasal steroids are used for environmental allergies. These are category C for use in pregnancy, which means they should be used only if the benefits outweigh potential risks. Although no human data are available regarding the intranasal use of topical steroids in pregnancy, beclomethasone (brand names Beconase and Vancenase) has been used for many years without apparent problems.
  • Over-the-counter Antihistamines
    Chlorpheniramine (Chlortrimetron), and diphenhydramine (Benadryl) are both category B. Common side effects are sleepiness and a dry mouth. Loratadine (Claritin) is also category B and may be less sedating than the older antihistamines.
  • Prescription Antihistamines
    While fexofenadine (Allegra) is category C, cetirizine (Zyrtec) falls into the B class and therefore is considered safer to use. Claritin and Zyrtec also may be less sedating than the older antihistamines. If you suffer from severe allergies, ask your practitioner for a recommendation.
  • Allergy Shots
    While there is no evidence that allergy shots are unsafe in pregnancy, experts don't recommend that expectant mothers start this treatment. If you're already on desensitization shots, you can usually remain at your current dose but not advance on the shot schedule until after delivery. Ask your physician for further information.
  • Decongestants
    This group of medications is used to treat colds or allergies. Pseudoephedrine (Sudafed), a category C drug, can be bought over the counter and is also present in antihistamines and cold remedies that say they contain a decongestant. These medications are not recommended for anyone who has high blood pressure, pregnant or not. If possible, avoid taking pseudoephedrine in the first trimester.
  • What allergy medications are safe during pregnancy?

From: Dr.Spock.com