Wait 6 months after Accutane to have a baby

Hi so I have a question. When I read the ipledge brochure. It says not to get pregnant one month after the last dose, but why does my Dr's request that I don't get pregnant until after 6 months of being off the medication. I read that Accutane doesn't stay long in the system but why are my Dr saying for me to wait 6 months. Is there any accurate time or when the medication will be out of the system? I think it'll be okay to try for a baby now since it's been over 4 months since i been off the medication.