Implantation: A circumvallate placenta is simply a 'accident of implantation and formation. Serendipity often causes some of the medical aberrations that are often seen. Causes of circumvallate placenta - What is a partial circumvallate placenta? What causes it? Implantation. A circumvallate placenta is simply a 'accident of implantation and formation. Serendipity often causes some of the medical aberrations that are often seen.