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 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 close to 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.
Placenta previa is estimated to occur in 1 in 200 pregnancies.
The causes and risk factors for placenta previa are:
- The fertilized egg implanted very low in the uterus, causing the placenta to form close to or over the cervical opening.
- The lining of the uterus (endometrium) has abnormalities such as fibroids or scarring (from previous previa, incisions, cesarean sections or abortions).
- The placenta formed abnormally.
- The pregnancy is multiple (i.e., twins or triplets). The chances of developing placenta previa are doubled for these pregnancies.
- The mother may have had several previous pregnancies. The chances of developing placenta previa are increased to 1 in 20 for women who have had 6 or more pregnancies.
- The mother smokes or uses cocaine. Smoking and cocaine use can increase the risk of this condition.
- The mother is older. The risk of developing placenta previa is 3 times greater in women over 30 years of age than in women under 20 years of age.
- The pregnancy has been conceived with the help of assisted reproductive technology, such as in-vitro fertilization.