Placenta Previa: Definition, Symptoms & Risks

Instructor: Sarah Lawson

Sarah has taught nursing courses and has a master's degree in nursing education.

Many potential complications can occur during pregnancy. One complication that can arise is placenta previa. Learn more about placenta previa and its associated symptoms and risks in this lesson.

What is Placenta Previa?

Pregnancy is a very complex process. As with any complex process, there are many problems that can arise. Placenta previa is just one of these potential problems. It occurs when the placenta partially or totally covers the cervix. The cervix is located at the lower end of the uterus and is the opening into the vagina or birth canal. Babies have to pass through the cervix to be born through the vagina. Placenta previa is a big problem because it causes the placenta to partially or totally block the baby's exit.

So what is this placenta that can cause so much trouble? It turns out it's very important -- the placenta is responsible for keeping the baby alive while in utero. It provides oxygen and nutrients to the growing baby and also removes waste from baby's blood. In most pregnancies, the placenta attaches at the top or side of the uterine wall. In placenta previa, the placenta attaches lower in the uterus.

Female reproductive system
Female reproductive system

Placenta previa occurs in approximately 1 in every 200 pregnancies. There are three different types of placenta previa:

  • Complete previa: cervical opening is completely covered by the placenta
  • Partial previa: cervical opening is partially covered by the placenta
  • Marginal previa: placenta extends just to the edge of the cervix

Normal placenta on left; Placenta previa on right


The main symptom of placenta previa is bright red, painless vaginal bleeding. This bleeding usually occurs in the second or third trimester and is usually the first sign of placenta previa. The bleeding may range from light to heavy. It often will resolve without treatment but will almost always return in the days or weeks following.

Some women will also experience premature contractions. Other symptoms that may indicate placenta previa include breech or transverse presentation of the baby and a uterus that measures large for gestational age.


The placenta will grow wherever the embryo implants in the uterus. If the embryo implants in the lower portion of the uterus, it is possible the placenta may grow over the cervix and cause a placenta previa.

Placenta previa is usually diagnosed during a second trimester ultrasound. It is possible, with a marginal previa, that expansion of the uterus as the baby grows will cause the placenta to move away from the cervix and resolve the situation. If the previa is complete or partial, it is unlikely it will move enough as the uterus expands. The later in a pregnancy that a placenta previa exists, the more likely it is that it will remain at the time of delivery.

Risk factors for having a placenta previa are:

  • Having scars in the lining of the uterus from previous surgery or cesarean sections (C-sections)
  • Having a large placenta, such as having a multiple pregnancy
  • Being of advanced maternal age (35 or older during pregnancy)
  • Having had previous pregnancies, especially if placenta previa occurred during them
  • Being non-Caucasian (race other than white)
  • Smoking cigarettes or using cocaine

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