Changes in your body during pregnancy sometimes alter the way you respond to allergens. There is no good way to predict whether your allergies will get better or worse or stay the same when you are pregnant. Fortunately, for about one out of three women, your body may noticeably quiet your immune system down (including your allergy symptoms) during a pregnancy. This is one of the many remarkable adjustments that your body can make so that your baby can grow, undisturbed, within you.

If you have seasonal allergies with symptoms that are bad enough to require taking allergy medication, it would be best if you could plan your pregnancy to avoid having your first trimester (first three months) overlap with allergy season.

This might help you to avoid taking allergy medications early in the pregnancy when the fetus is developing its organs, but allow you to take allergy medications for severe symptoms later in pregnancy. Of course, it is not always realistic for you to determine the timing of your pregnancy.

Medications are graded by classes A through D to indicate their level of safety if used during pregnancy:

 
• Class A is safest. These medications have been proven to be safe in pregnant women.  
• Class B medications are considered safe during pregnancy, although definitive study evidence in humans is lacking.  
• Class C medications are potentially harmful according to animal studies, although no studies in humans are available.  
• Class D medications have been shown to be harmful to human fetuses.

Among allergy medications, the best score available is Class B. Some allergy medicines may or may not be safe enough to deserve a Class A score, but large studies would be needed to prove this, and these studies have not been done.

Asthma can have a significant effect on pregnancy, and controlling asthma during pregnancy is an important priority. Asthma inhalers range in their safety ratings, and most are assigned to Class B or Class C. Even if your inhaler has a rating of Class C, you should use it without restriction during pregnancy if your doctor advises you to do so. You should focus first on your asthma control, rather than your inhaler ratings. Several studies report that poor asthma control results in low birth-weight babies and preterm births. This may be related to the mother’s chemical or hormonal reaction to physical stress, or subtle shifts in oxygen and carbon dioxide gases within the bloodstream during an asthma flare. Asthma prevention by avoiding your triggers is especially important during pregnancy.

Most, but not all antihistamines have Class B ratings for pregnancy safety. Popular antihistamines include over-the-counter medications such as chlorpheniramine (Chlor-Trimeton) or loratadine (Claritin), or prescription antihistamines such as cetirizine (Zyrtec). If cost is a consideration, chlorpheniramine is often recommended because it causes somewhat less drowsiness than other inexpensive antihistamines. Fexofenadine (Allegra) has a class C rating in pregnancy.

Cromolyn sodium nasal spray (Nasalcrom) is another option with Class B rating that can be used to control allergy symptoms during pregnancy. It is helpful for prevention but does not provide immediate relief.

Allergy shots can be used during pregnancy, because there is no evidence that allergy shots affect the fetus. If you have never received allergy shots, you should wait to start until after your baby is born.

Most other allergy medications (including corticosteroid nose sprays and oral decongestants) are class C. Sometimes a class C medication is appropriate for short-term treatment, depending on the severity of your symptoms and the stage of your pregnancy. Do not take any medication without consulting your obstetrician. This includes herbs, dietary supplements and complementary therapies, some of which can harm the developing fetus.