Does implantation bleeding happen every month in pregnancy? How many months it will last?

Just once. Implantation bleeding happens with implantation of the tiny embryo into the lining of the uterus. It happens just once, during the first weeks of pregnancy. There is another type of bleeding, called decidual bleeding, that can happen more than one time during the first trimester. One should be sure to tell her ob-gyn doctor about any bleeding episodes.