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Placenta Previa in Pregnancy (cont.)

Placenta Previa Causes

A number of factors can increase the likelihood that the placenta will be located in the lower part of the womb and potentially cover the cervical opening.

Scar tissue in the upper regions of the uterus can promote growth of the placenta in the relatively unscarred lower segment of the uterus. Scarring of the tissues in the upper uterus can be a result of the following:

  • prior Cesarean deliveries (placenta previa occurs in 10% of women who have had four or more Cesarean deliveries);

  • prior D&C procedures (curettages) for miscarriages or induced abortions; and

  • any surgery or instrumentation of the uterine cavity.

In some cases, placenta previa occurs because the placenta grows larger to compensate for decreased function (lowered ability to deliver oxygen and/or nutrients) or a need for greater function. This need for a larger placental area can increase a woman's risk of developing placenta previa. Examples include:

  • multiple gestation (twins, triplets, etc.);

  • cigarette smoking; and

  • living at high altitude.

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 are at greater risk for this complication in subsequent pregnancies. Asian women also have a slightly greater risk of placenta previa than women of other races, although the reason for this is unclear. Further, it has been noted that women carrying male fetuses are at slightly greater risk for placenta previa than are women carrying female fetuses.

Since the placenta normally migrates away from the cervical opening as pregnancy progresses, women in the earlier stages of pregnancy are more likely to have placenta previa than are women at term. Up to 6% of women between 10 and 20 weeks' gestation will have some evidence of placenta previa on ultrasound examination, but 90% of these cases resolve on their own as the pregnancy progresses.

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