Placenta previa

Definition
The placenta is an organ that develops in the womb during pregnancy to nourish the fetus and help it discard waste. Normally, the placenta is attached safely to the top portion or sides of the uterus. Placenta previa occurs when the placenta attaches incorrectly to the lower uterus and blocks the cervix (the tip of the uterus that extends into the vagina). Near the end of pregnancy, the lower uterus softens and thins, and the cervix eventually opens or dilates to allow the baby to move down the birth canal. With placenta previa, these changes cause the placenta to prematurely separate from the uterine wall, resulting in bleeding.

Causes/associated factors
Placenta previa is described as total if the cervix is completely covered by the placenta, partial if only some of the cervix is covered by the placenta, and marginal if the placenta is located at the edge of the cervix but does not cover it. Your doctor may also describe placenta previa as the percent of the cervix that's covered by the placenta, such as 30, 50, 75 or 100 percent covered. The amount of cervix that's covered by the placenta may increase or decrease as the cervix dilates.
You may also have a low-lying placenta in which the placenta is located in the lower part of the uterus but doesn't touch the edge of the cervix. This isn't really placenta previa since there's no contact with the cervix. As the lower uterine segment stretches and the uterus enlarges later in pregnancy, the placenta usually moves away from the cervix.

The exact cause of placenta previa is unknown, but risk factors include:

previous cesarean section, uterine surgery or abortion
advanced maternal age
multiple pregnancy (twins or more)
more than four previous pregnancies
placenta previa in a previous pregnancy
smoking
Before 20 weeks of pregnancy, up to six out of every 100 pregnant women have some degree of placenta previa. In most cases, however, the placenta migrates to a higher position as the uterus enlarges, usually by 30 weeks of pregnancy. Placenta previa persists beyond 30 weeks in only about one out of every 200 pregnancies.


Signs/symptoms
The first symptom of placenta previa is typically bleeding, which is usually bright red and painless. It can begin as early as 20 weeks, but usually occurs later in pregnancy when the lower uterine segment is changing significantly. Initially, the bleeding is often light (although heavy bleeding is possible). The bleeding often stops spontaneously, but will typically recur. The bleeding may first be noticed after an event such as a pelvic exam or sex, or it may first occur during labor. In some cases, it may occur while you're at rest without any apparent precipitating factors. Some women may also have cramping or contractions.

Diagnosis
If your doctor suspects placenta previa, a vaginal exam is not recommended. In the rare cases when it's necessary, the exam is done in an operating room where an emergency C-section can be done in case severe bleeding develops.
Placenta previa can often be diagnosed with an abdominal ultrasound (using high-frequency sound waves directed through the abdomen to create an image of the uterus and the placenta). If the diagnosis is uncertain after this exam, a transvaginal ultrasound (directing the sound waves through the vagina) may be necessary.

Typically, placenta previa is diagnosed during a routine ultrasound in the second trimester, before symptoms occur. In this situation, the ultrasound is repeated between weeks 30 and 32 to determine whether the condition still exists or the placenta has moved away from the cervix on its own.


Treatment
The treatment of placenta previa depends on the amount of bleeding and your baby's gestational age. Additional tests may be needed to help your doctor decide the best time for delivery. Ultrasound is used to determine your baby's gestational age, and amniocentesis (a procedure in which amniotic fluid, the fluid that surrounds and protects the fetus in the womb, is withdrawn for examination) is done to help the doctor evaluate your baby's lung maturity. The delivery will most often be done by C-section.
If you're not bleeding or the bleeding is not severe, and you're at least 36 weeks pregnant, delivery is usually done as soon as possible. Before week 36, your doctor will try to postpone delivery until your baby matures. If you're bleeding, you'll be hospitalized until the bleeding has stopped, and you may need to stay in the hospital until you deliver your baby. Your doctor will probably recommend bedrest, and you and your baby will be closely monitored. If labor begins early, you may be given medication to try to stop it. If you're less than 34 weeks pregnant, you may be given steroid medications to speed your baby's lung development.

If you're not bleeding or the bleeding has stopped, and you live close to a hospital that can adequately care for you and your baby, you may be allowed to go home where you'll continue the bedrest. You'll need someone with you at all times, immediate access to a telephone, and transportation to the hospital in case bleeding recurs. At least weekly, you'll return to the hospital or clinic for a nonstress test (the use of ultrasound to measure fetal heart rate accelerations, usually associated with fetal movement) or other tests to help evaluate your baby's health. Your doctor may ask you to monitor your baby's activity daily with a kick count.

If you're having uncontrolled, heavy bleeding, you'll need an emergency C-section -- regardless of your baby's gestational age. Blood transfusions may be needed to replace lost blood.


Complications
Placenta previa can result in serious complications for both mother and baby. Serious bleeding can be life-threatening, but medical advances in the diagnosis and treatment of placenta previa have significantly reduced that threat. Because of blood loss, both mother and baby can develop anemia (a condition marked by a decreased number of red blood cells or hemoglobin -- a protein in red blood cells that contains iron; the red blood cells carry oxygen throughout the body).
Other possible complications include:

premature rupture of the membranes
preterm labor
postpartum hemorrhage
postpartum infection
For the baby, premature birth is the major complication. In addition, the baby may experience poor growth prior to birth and present as breech (feetfirst) or transverse (crosswise) instead of headfirst. There's also an increased risk of birth defects.


Prevention
If you smoke or use cocaine, quitting will reduce the risk of placenta previa and help ensure a healthier pregnancy. It's also important to seek early and regular prenatal care, which may reduce the risk of complications from placenta previa.