Placenta Previa

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Medical Author: Melissa Conrad Stöppler, MD
Medical Editor: William C. Shiel, Jr., MD, FACP, FACR

• What is placenta previa?
• What are the types of placenta previa?
• Who is at risk for placenta previa?
• What causes placenta previa?
• What are the symptoms of placenta previa?
• How is placenta previa diagnosed?
• What is the treatment for placenta previa?
• What are possible complications of placenta previa?
• Can placenta previa be prevented?
• What is the prognosis (outlook) for placenta previa?
• Placenta Previa At A Glance

What is placenta previa?

Placenta previa is the most common cause of painless bleeding in the later stages of pregnancy (after the 20th week). The placenta is a temporary organ that joins the mother and fetus and transfers oxygen and nutrients from the mother to the fetus. The placenta is disk-shaped and at full term measures about seven inches in diameter. The placenta attaches to the wall of the uterus. Placenta previa is a complication that results from the placenta implanting either near to, or overlying, the outlet of the uterus (womb).

Because the placenta is rich in blood vessels, if it is implanted near the outlet of the uterus (the opening of the cervix), bleeding can occur when the cervix dilates or stretches.

What are the types of placenta previa?

The types of placenta previa include:

• Complete placenta previa occurs when the placenta completely covers the opening from the womb to the cervix.
  
  
• Partial placenta previa  occurs when the placenta partially covers the cervical opening
  
  
• Marginal placenta previa occurs when the placenta is located adjacent to, but not covering, the cervical opening.

The term low-lying placenta or low placenta has been used to refer both to placenta previa and marginal placenta previa. Sometimes, the terms anterior placenta previa and posterior placenta previa are used to further define the exact position of the placenta within the uterus as defined by ultrasound examinations.

Who is at risk for placenta previa?

Placenta previa is found in approximately four out of every 1000 pregnancies beyond the 20th week of gestation. Asian women are at a slightly greater risk for placenta previa than are women of other races, although the reason for this is unclear. It has also been observed that women carrying male fetuses are at slightly greater risk for placenta previa than are women carrying female fetuses.

The risk of having placenta previa also increases with increasing maternal age and with the number of previous births. Women who have had placenta previa in one pregnancy also have a greater risk for having placenta previa in subsequent pregnancies.

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