Products containing Isotretinoin are generally not used in women of child bearing age unless absolutely essential i.e. situations where acne are (a) severe and (b) do not respond to any other treatment. The drug is teratogenic i.e. it causes malformations of the unborn baby. Your doctor is absolutely correct in advising you not to conceive for at least eight weeks after stopping the therapy. Since the drug is deposited in the fat tissues, to be doubly and absolutely sure it would be better to avoid pregnancy for 3 months. Regulatory authorities in advanced countries insist on contraception for one month after stopping treatment; hence three months is more than adequate both for clinical reasons and peace of mind.