The allergy medication loratadine (Claritin, others) is currently considered a category B drug. This indicates that animal studies haven't shown any risks to a fetus whose mother takes the drug, and there have been no reports of human reproductive problems associated with the drug. Although category B drugs appear to be safe, they haven't been adequately tested during human pregnancy to offer any guarantees.
In fact, few drugs have actually been proved safe during pregnancy. Drugs typically aren't studied in pregnant women because of the possible effects on the fetus. Until more is known, doctors must make judgments using the best available scientific evidence. It's best to exercise caution before taking allergy medications or any other drugs during pregnancy. Work with your doctor to weigh the severity of your allergy symptoms against the possible risks to your baby from taking allergy medications.
If possible, limit your exposure to anything that triggers your allergy symptoms. This may decrease your need for allergy medications. Also, keep in mind that nasal congestion often gets worse during pregnancy due to hormone changes. This occurs even in women who don't have allergies.