D.W.
I just responded to another post to be careful and take what you read about health-related topics online with a grain of salt. Everyone has different motivations when putting in formation online.
A recent Harris study indicated that there is almost as much poor, inaccurate misinformation online as there is good, credible information about health-related topics.
Here is the FDA approved information on Imitrex. According to the label (what the FDA has approved the product for), it carries Pregnancy Category C rating. That means that data hasn't been submitted to the FDA to seek the approval. But this is an exact quote from the label, "IMITREX should be used during pregnancy only if the potential benefit justifies the potential risk to the fetus."
Your OB/GYN won't allow you to take it without fully assessing the risk to you and your baby first. They have too much liability.
Don't Google the subject - I just did, and the sites it takes you to aren't very medically credible. Instead, go to Google.com/scholar. The data is very clinical, but I found the following article from the Annals of Pharmacology that was published in 2004 and shows no increased risk of birth defects using Imitrex vs. not: http://www.theannals.com/cgi/content/abstract/38/10/1726
Still, ask your doctor, confirm with your pharmacist, and don't be afraid to question if you don't feel right about it.