Placenta previa definition and facts
Placenta previa is diagnosed by an ultrasound examination.
- Placenta previa is the attachment of the placenta to the wall of the uterus in a location that completely or partially covers the uterine outlet (opening of the cervix).
- Bleeding after the 20th week of gestation is the main symptom of placenta previa.
- An ultrasound examination is used to establish the diagnosis of placenta previa.
- Treatment of placenta previa involves bed rest and limitation of activity. Tocolytic medications, intravenous fluids, and blood transfusions may be required depending upon the severity of the condition.
- A Cesarean delivery is required for complete placenta previa.
- Other complications of pregnancy can be associated with placenta previa, but the majority of women deliver healthy babies.
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 (womb). Placenta previa is a complication that results from the placenta implanting either near to, or overlying, the outlet of the uterus (the opening of the uterus, the cervix).
Because the placenta is rich in blood vessels, if it is implanted near the outlet of the uterus, bleeding can occur when the cervix dilates or stretches.
What are the signs and symptoms of placenta previa?
Bleeding is the primary symptom of placenta previa and occurs in the majority (70%-80%) of women with this condition.
- Vaginal bleeding after the 20th week of gestation is characteristic of placenta previa.
- Usually the bleeding is painless, but it can be associated with uterine contractions and abdominal pain.
- Bleeding may range in severity from light to severe.
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 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 for miscarriages or induced abortions) of the uterine cavity, or any type of surgery involving the uterus.
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.
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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 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.
How is placenta previa diagnosed?
An ultrasound examination is used to establish the diagnosis of placenta previa. Either a transabdominal (using a probe on the abdominal wall) or transvaginal (with a probe inserted inside the vagina but away from the cervical opening) ultrasound evaluation may be performed, depending upon the location of the placenta. Sometimes both types of ultrasound examination are necessary.
It is important that the ultrasound examination be performed before a physical examination of the pelvis in women with suspected placenta previa, since the pelvic physical examination may lead to further bleeding.
What is the treatment and management for placenta previa?
Treatment of placenta previa depends upon the extent and severity of bleeding, the gestational age and condition of the fetus, the position of the placenta and fetus, and whether the bleeding has stopped.
Cesarean delivery (C-section) is required for complete placenta previa and may be necessary for other types of placenta previa. A Cesarean delivery is usually planned for women with placenta previa as soon as the baby can be safely delivered (typically after 36 weeks' gestation), although an emergency Cesarean delivery at any earlier gestational age may be necessary for heavy bleeding that cannot be stopped after treatment in the hospital (see below).
Women who are actively bleeding or who have bleeding that cannot be stopped will be admitted to the hospital for further care. If there has been little or no bleeding or the bleeding has stopped, bed rest at home may be prescribed. Home care is not always appropriate, and women who remain at home must be able to access medical care immediately should bleeding resume. Women with placenta previa in the 3rd trimester of pregnancy are advised to avoid sexual intercourse and exercise and to reduce their activity level.
Women with placenta previa who experience heavy bleeding may require blood transfusions and intravenous fluids. In some cases, tocolytic drugs (medications that slow down or inhibit labor), such as magnesium sulfate or terbutaline (Brethine) are necessary. Corticosteroids may be given to enhance lung development in the fetus prior to Cesarean delivery.
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What are possible complications of placenta previa?
Placenta previa can be associated with other abnormalities of the placenta or of the umbilical cord. Some studies have shown a reduction in fetal growth associated with placenta previa, and the presence of the placenta in the lower part of the uterus makes breech or abnormal presentation of the fetus more likely.
The bleeding of placenta previa can increase the risk for preterm premature rupture of the membranes (PPROM), leading to premature labor.
Placenta accreta is a serious complication that occurs in 5% to 10% of women with placenta previa. Placenta accreta results when the placental tissue grows too deeply into the womb, attaching to the muscle layer, resulting in difficulty separating the placenta from the wall of the uterus at delivery. This complication can cause life-threatening bleeding and commonly requires hysterectomy at the time of Cesarean delivery.
Lastly, as with other complications of pregnancy, placenta previa can have a significant emotional impact on the pregnant woman.
Can placenta previa be prevented?
Placenta previa cannot typically be prevented. In some cases, risk factors for the development of placenta previa can be eliminated (such as smoking cessation).
What is the prognosis for a woman with placenta previa?
The majority of women with placenta previa in developed countries will deliver healthy babies, and the maternal mortality (death) rate is less than 1%. In developing countries where medical resources may be lacking, the risks for mother and fetus may be higher.
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Medically Reviewed on 9/13/2019
Bakker, R, MD. Placenta Previa. Medscape. Updated: May 12, 2016.