Okay to use Miralax??

I have been dealing with awful constipation. My doctor recommended Miralax but I saw on-line that Miralax is a a category C for medication for pregnant women. I had no idea and now I'm worried because i have been using it a couple times a week. Anyone have a doctor that suggested Miralax??? I have an OB appointment Wednesday and I will talk with the them then. Just wondered if anyone has been using it.