Of course, we have a pair of everything (except X and Y). So if the mutation is simply that a function does not work (recessive) but only one functional gene is needed, then there is a chance that even with a significant mutation there will not be a miscarriage. However, if that non-functional gene is now carried by a mother and father, it increases the risk that the couple will have more miscarriages than normal. Miscarriages would be a way of weeding out the incidence of that bad gene.