I was able to find a couple of papers on PubMed which discussed this topic.
Pregnancy outcomes during treatment with interferon beta-1a in patients with multiple sclerosis.
Neurology. 2005 Sep 27;65(6):802-6.
The reproductive effects of beta interferon therapy in pregnancy: a longitudinal cohort.
Neurology. 2005 Sep 27;65(6):807-11.
It appears from these two papers that the news may not be too good. However, I wish you the best and hope that your baby is healthy. The best thing to do may be to keep your doctor informed of your condition and aware of your interferon use. Short of minimizing any other risk factors, such as caffeine usage or exposure to second hand smoke, I'm not sure what else can be done. Hopefully your doctors will have more ideas.
I also found this statement on the National MS Society's site.
The FDA issued guidelines requiring the manufacturers of Avonex, Betaseron, and Rebif to develop pregnancy registries to monitor women who have taken one of these drugs within a week of becoming pregnant or while they were pregnant.
Here's a link to a PDF describing Biogen's registry