Placenta previa

Definitions & Key takeaways

Placenta previa is a pregnancy complication in which the placenta implants in the lower uterus and partially or fully covers the internal cervical os, making vaginal delivery difficult or impossible. It can cause heavy vaginal bleeding and a serious threat to both the mother and the fetus. Symptoms include painless vaginal bleeding in the third trimester, and some risk factors are previous placenta previa, multiple gestation, and uterine fibroids.

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Placenta previa means “placenta first,” because the placenta is the first thing within the uterine cavity. In this condition, the placenta implants in the lower uterus, close to or even covering the uterine opening, called the internal cervical os, and it can therefore easily bleed, which usually happens after 20 weeks of gestation.
Normally the placenta implants in the upper uterus, and it's unclear why it implants in the lower uterus. One hypothesis is that the placenta implants lower down when the endometrium in the upper uterus is not well vascularized.
In fact, endometrial damage from things like a previous cesarean section, an abortion (which could be induced or spontaneous), uterine surgery, and multiparity or multiple pregnancies can decrease vascularization and increase the risk of placenta previa.
In other cases, risk factors for placenta previa include having multiple placentas or a placenta with a larger than normal surface area, which can both happen with twins or triplets, as well as maternal age of 35 years or more, intrauterine fibroids, and maternal smoking.
Placenta previa is classified by how close the placenta is to the cervical os, it can be complete where the placenta completely covers the cervical os; partial where the placenta partially covers the cervical os; or marginal where the edge of the placenta extends to within 2 cm of the cervical os.
As the pregnancy progresses, the lower uterine segment grows, and if the placenta’s in the lower uterus, this growth disrupts the placental blood vessels, which can cause bleeding.
This usually a sudden onset of painless bright red bleeding that typically happens after 20 weeks gestation. The amount of bleeding can vary, and it can be intermittent or continuous, sometimes increasing during labor because of uterine contractions and cervical dilation.
Complications on the maternal end are related to the amount of blood loss, and fetal complications can include hypoxia and preterm delivery.
Placenta previa is also associated with placenta accreta, which is where the placenta invades the myometrium and becomes inseparable from the uterine wall.
Placenta previa is often diagnosed during prenatal ultrasounds, but occasionally it’s not recognized until cervical changes during labor cause bleeding to happen.
For treatment or management, the main goal is to plan for and prevent preterm birth, and to manage maternal bleeding. Corticosteroids can be sometimes given to help enhance fetal lung maturity.
For minor maternal bleeding, bed rest is usually sufficient, but for major bleeding, blood products and intravenous fluids might be used to support hemodynamic stability.
In some severe cases, where the maternal hemodynamics are worrisome and the fetal heart rate tracing shows signs of fetal hypoxia, an immediate cesarean section can be performed.
Alright, as a quick recap, placenta previa describes a placenta that’s in the lower uterus instead of the upper uterus. The internal cervical os might be covered completely, partially, or marginally, and this can bleeding that’s painless and bright red, and can increase the likelihood of a preterm delivery and fetal hypoxia.
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