Which medicine is the safest for treating pinworm in pregnancy ?

None in pregnancy: The medications used for the treatment of pinworm are mebendazole, pyrantel pamoate, or albendazole. However, safety of these drugs has not been studied for pregnant women. If the infection is compromising the pregnancy (i.e. weight loss, sleeplessness) then treatment can be considered, but should be withheld until the 3rd trimester when risk to fetus is lower. Ref: http://www.cdc.gov