Linea nigra, a dark line that extends from the umbilicus to the mons pubis - Placenta previa is abnormally low implantation of the placenta

- Linea nigra, a dark line that extends from the umbilicus to the mons pubis, commonly appears during pregnancy and disappears after pregnancy.

- Implantation in the uterus occurs 6 to 10 days after ovum fertilization.

- Placenta previa is abnormally low implantation of the placenta so that it encroaches on or covers the cervical os.