@Kat919 - I agree that if you are really suffering and something that is generally considered safe will help, then it's okay to take medicine during pregnancy. But not all pregnancy complaints respond to medicine, and antihistamines won't help with all causes of congestion. Sometimes, you just have to breathe through your mouth and wait for it to end!
Some category C medicines may also be pretty safe during pregnancy. My doctor told me that pseudophedrine (the stuff in Sudafed and all the good cold medicines) is category C, but that pregnant women have taken it for years without problems. As always, ask your doctor, but I have a friend whose home birth midwife even told her to take Sudafed!