Pregnancy: Placenta Previa (cont.)

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

What causes placenta previa?

The placenta may be located in the lower part of the uterus either covering or adjacent to the cervical outlet for a number of reasons. The placenta normally migrates away from the cervical opening as the pregnancy progresses, so women in the earlier stages of pregnancy are more likely to have placenta previa than are women at term. Although up to 6% of women between 10 and 20 weeks' gestation will have some evidence of placenta previa on ultrasound examination, 90% of these cases resolve on their own as the pregnancy progresses.

Placenta previa that persists beyond the 20th week of gestation can be due to abnormalities of the uterus that promote attachment of the placenta in the lower regions of the uterus or to factors that require an increased size of the placenta.

Uterine factors

Uterine factors that can predispose to placenta previa include scarring of the upper lining tissues of the uterus. This can occur because of prior Cesarean deliveries, prior instrumentation (such as D&C procedures or curettages for miscarriages or induced abortions) of the uterine cavity, or any type of surgery involving the uterus.

Placental factors

When the placenta must grow larger to compensate for decreased function (lowered ability to deliver oxygen and/or nutrients), there is an increased chance of developing placenta previa since the surface area of the placenta will be larger. Examples of situations in which there is need for greater placenta function and a resultant increase in risk for placenta previa include multiple gestation, cigarette smoking in the mother, and living at high altitude.

Medically Reviewed by a Doctor on 1/10/2014

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