Pegasys is not used as a monotherapy in hep C patients. If he is taking it with ribavirin that would be for hepatitis C. People with pregnant partners should not use ribavirin during their partners pregnancy.
Interferon alone is sometimes used for hairy cell leukemia, kidney cancer and melanoma. It is not used for liver damage as a monotherapy since it was proven that it does not help the liver recover from damage due to any reason. It is critical that you find out if he is using ribavirin and if he uis speak to your ob/gyn.
Interferon (which is what pegasys is) has been around a long time and the drug can be used to treat other things, including some cancers. If your partner is indeed taking these drugs then you must use a condom as the drugs can be transmitted to you and the baby though semen. If you partner does have Hep C the chances are very slim that it can be passed through normal sexual intercourse, however for the safety of you and your baby I strongly recommend you use condoms ALWAYS. I hope that helps.
hear is a link to pegasys web page
http://www.pegasys.com/patient/index.html