The placenta is a bed of cells formed inside the uterus (womb) during pregnancy. The purpose of the placenta is to carry nourishment and oxygen from the mother to the fetus and to carry waste products and carbon dioxide from the fetus to the mother through the umbilical cord.
The placenta is usually formed along the upper part of the uterus, allowing enough space for the fetus to grow. In placenta previa, the placenta starts forming very low in the uterus or even over the cervix (the opening of the uterus that leads to the vagina). This obstruction impairs normal vaginal delivery of the baby at birth.
There are 3 types of placenta previa:
- complete placenta previa: The internal cervical opening is completely covered by the placenta.
- partial placenta previa: The internal cervical opening is partially covered by the placenta.
- marginal placenta previa: The placenta is at the edge of the internal cervical opening.
A “low-lying” placenta does not cover or touch the cervix, but is within 2 centimetres of the opening. Placenta previa is estimated to occur in 1 in 250 pregnancies.