Final answer:
Common risk factors associated with abnormal placental implantation include placenta previa, embryo implantation outside the uterus or in an unfavorable location within the uterus, and infections or exposures to substances like alcohol and drugs.
Step-by-step explanation:
Abnormal placental implantation can be caused by several risk factors. One common risk factor is placenta previa, where the placenta partially or completely covers the cervix. This condition can lead to antepartum hemorrhage, which is profuse vaginal bleeding after week 24 of pregnancy but before childbirth. Another risk factor is the implantation of the embryo outside the uterus or in an unfavorable location within the uterus, which can result in complications such as placenta previa. Lastly, certain infections like rubella and exposure to substances like alcohol and drugs can also increase the risk of abnormal placental implantation and related complications.