If I started taking prevpack antibiotic on early pregnancy stage but didn't know I was pregnant, could it cause miscarriage or fetus abnormalities?

Research shows. That there is a small increased risk of miscarriage with the use of clarithromycin, one of the components of the prevpack, early in pregnancy, but no significant increase in risk of fetal malformations. Reference here: http://www.Plosone.Org/article/info%3adoi%2f10.1371%2fjournal.Pone.0053327 i would suggest discussing these concerns with your physician, who may choose to do an early u/s to eval.