Placenta previa occurs when the placenta abnormally ___ in the ___ segment of the uterus near or over the ___ instead of attaching to the ___. The abnormal implantation results in ___ during the ___ trimester of pregnancy as the cervix begins to ___ and ___. Classified into ___ types of dependent on the degree to which the ____ is ___ by the placenta. ___: the cervical os is completely covered by the placental attachment___: the cervical os is only partially covered by the placental attachment___: the placenta is attached in the lower uterine segment but does not reach the cervical os___: the exact relationship of the placenta to the internal os has not been determine