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Tests and diagnosis
By Mayo Clinic staffPlacenta previa is diagnosed through ultrasound, either during a routine prenatal appointment or after an episode of vaginal bleeding. Placenta previa is nearly always detected before a woman or her baby is in significant danger.
Diagnosis before 20 weeks of pregnancy
It's not unusual to detect a low-lying placenta or to see the placenta covering the cervix during a routine midpregnancy ultrasound. Most of these cases resolve on their own before delivery, as the uterus grows and the placenta migrates away from the cervix. You may need additional ultrasounds to track the position of your placenta. The longer placenta previa persists, the more likely it will be present at delivery.
Diagnosis after 20 weeks of pregnancy
Your health care provider may detect placenta previa later in pregnancy during an ultrasound for an unrelated reason. At this stage of pregnancy, however, vaginal bleeding is usually the tip-off.
If you experience vaginal bleeding during the second or third trimester, call your health care provider right away. You'll likely need to go to your doctor's office or the hospital to determine the cause of the bleeding. In most cases, your health care provider can use an abdominal ultrasound to quickly confirm or rule out placenta previa.
A definitive diagnosis may require a combination of abdominal ultrasound and transvaginal ultrasound, which is done through a wand-like device (transducer) placed inside your vagina. Your health care provider will closely monitor the location of the transducer in your vagina to prevent any bleeding. Rarely, magnetic resonance imaging (MRI) may be used to diagnose placenta previa.
If your health care provider suspects that you may have placenta previa, he or she will avoid routine vaginal exams to reduce the risk of heavy bleeding. You may need additional ultrasounds or, rarely, an MRI to determine the exact location of your placenta before delivery. Your baby's heartbeat may be tracked as well.
Related conditions
Two uncommon conditions are often grouped with placenta previa because they can cause vaginal bleeding in the late second or third trimester. If you have vaginal bleeding late in your pregnancy, your health care provider will also consider these conditions before making a diagnosis:
- Placental abruption. Rarely, the placenta separates from the uterus before birth. This can deprive the baby of oxygen and nutrients and cause heavy bleeding that may be dangerous for both mother and baby.
- Vasa previa. The umbilical cord usually develops in the center of the placenta. If the umbilical cord attaches to the placenta in an unusual way, a portion of the blood vessels normally inside the umbilical cord may be left unprotected. If these unprotected blood vessels cross the cervix, it's known as vasa previa. If these blood vessels rupture, the baby faces life-threatening bleeding.
- Lockwood CJ, et al. Clinical manifestations and diagnosis of placenta previa. http://www.uptodate.com/hom. Accessed April 17, 2009.
- Sakornbut E, et al. Late pregnancy bleeding. American Family Physician. 2007;75:1199.
- Zeltzer JS. Vaginal bleeding in late pregnancy. In: Rakel RE, et al. Conn's Current Therapy 2008. 60th ed. Philadelphia, Pa.: Saunders Elsevier; 2008. http://www.mdconsult.com/das/book/body/132444171-3/831220627/1621/508.html#4-u1.0-B978-1-4160-4435-2..50256-9_4080. Accessed April 17, 2009.
- Lockwood CJ, et al. Management of placenta previa. http://www.uptodate.com/hom. Accessed April 17, 2009.
