Placenta Previa

Definition

Placenta previa is an abnormal condition of pregnancy in which the placenta is attached to the lower section of the uterus, partially or completely covering the cervix. It occurs in about 0.5% of pregnancies.

Description

The placenta is a hormone-producing fetal organ, rich in blood vessels, that connects the baby to the mother via the umbilical cord. It begins to develop along with the embryo right after conception. The placenta normally implants high on the uterine wall and securely attaches into the wall through finger-like projections. The umbilical cord is created by the interweaving of two arteries and one vein that connect the placenta to the fetus. The nutrients and oxygen from the mother pass through the placenta and into the fetus. Carbon dioxide and waste products excreted by the fetus pass through the placenta and into the mother's circulation for removal. The placenta functions as a lifeline for the fetus.

In placenta previa, the placenta has attached itself towards the bottom of the uterus, near or on the cervix. Its usual implantation site is high up on the uterine wall. During a vaginal birth the cervix thins and opens sufficiently for the fetus to pass through the cervix, into the birth canal, and out the mother's vagina. As the cervix begins to dilate during labor, the force on the low-lying placenta causes tearing and subsequent bleeding. Excessive bleeding, or hemorrhage, can be dangerous for both the mother and fetus. If the placenta tears, the fetus is deprived of nutrients and oxygen and can suffer brain damage or even death.

In some pregnancies, the low placement may not provide a sufficiently large area for good exchange of nutrients and gases. This may impede fetal growth. In most cases of placenta previa, the condition becomes a concern towards the end of the pregnancy, often around 30 weeks gestation. At this point the uterus starts to undergo changes in preparation for labor and delivery. It is when these changes occur that the placenta may begin to tear and bleed. Infants born to mothers with placenta previa also have a greater risk of respiratory distress syndrome. In a study published in October of 2000, researchers found that mothers carrying a male fetus are at greater risk of placenta previa than those carrying a female fetus.

There are four degrees of placenta previa:

• Low-lying implantation. The placenta lies abnormally low on the uterine wall, but is not yet approximating the cervix.
• Marginal implantation. The placenta is within 2 centimeters of the internal cervical os.
• Partial previa. The placenta covers part of the cervix. The distinction between partial and complete previa is somewhat unclear while the cervix is still closed.
• Total previa. The placenta completely covers the cervical os.

Causes and symptoms

The exact cause of placenta previa is unknown. However, contributing factors include:

• Uterine shape abnormality.
• Increased parity, i.e. the mother has been pregnant before.
• Previous cesarean births.
• Older maternal age. Women over the age of 35 have an increased risk of placenta previa by 4.7 times. When the mother is over 40, the risk rises to 9 times.
• Previous dilation and curettage of the uterus.
• Multiple gestation (twins, triplets, etc.).

• Previous placenta previa.
• Cigarette smoking.

Placenta previa is characterized by painless vaginal bleeding that often starts abruptly. The bleeding may continue, or it may stop as abruptly as it started. By the time the woman is seen by her obstetrical provider, there may be some spotting, or perhaps no bleeding at all. Even if the bleeding has stopped, placenta previa is an emergency situation and the mother needs to be seen right away. Bleeding indicates that the placenta has begun to tear. The mother is now at risk of hemorrhage if no intervention is made. The fetus may be compromised as the level of oxygen available to it has changed.

Diagnosis

Most pregnant women undergo at least one routine ultrasound during their pregnancy. During the ultrasound the placement and position of the placenta is identified. When a low-lying placenta is detected, the degree to which the placenta covers the cervical os is described in percentages. For example, a complete placenta previa is 100%. Once placenta previa has been diagnosed, the pregnancy is considered high risk. However, the position of the placenta can change as the uterus grows, and so periodic ultrasounds may be ordered. A transvaginal ultrasound may be ordered following an abdominal ultrasound to more accurately assess how low the placenta is lying. There is a false-negative and false-positive risk of 7% with abdominal ultrasound. For this condition, transvaginal ultrasound appears to be far more reliable. In addition, the placenta is able to creep upwards over time.

If a woman experiences sudden, painless, bright red bleeding at any point in the pregnancy, she should be seen right away, even if the bleeding has stopped. An ultrasound will usually be done in order to reassess the position of the placenta, and to evaluate the fetus. A manual examination of the cervix is not done, as this could disturb the placenta.

Treatment

The treatment plan will depend on the gestational age, the severity of the bleeding, and the risks to mother and fetus. If the fetus is sufficiently mature and the pregnancy is near term, immediate cesarean birth may be suggested. If it is too early for the fetus to survive outside the womb, and the mother's condition is stable, the mother may be placed on bed rest in the hospital with medications to prevent uterine contractions. Close monitoring of the fetus and mother will continue. If there has been no bleeding for a few days, the mother may be sent home, and may be prescribed medication to improve the fetus' lung maturity so that if a preterm birth is necessary, the fetus has a better chance for healthy survival. The position of the placenta will determine if a vaginal birth is possible, or if the safest delivery will be by cesarean section.

Prognosis

Prognosis for mother and fetus have significantly improved with accurate imaging technology that identifies the condition in advance and allows for proper high-risk management of the pregnancy. A planned cesarean birth rather than unexpected, profuse bleeding at the time of labor is a part of placenta previa management. The prognosis for the fetus depends on how well developed it is at the time of delivery, and whether the bleeding caused any significant oxygen deprivation. The mother has an excellent prognosis unless hemorrhage occurs and is not well managed.

Health care team roles

The radiologic technologist usually performs the ultrasound, but the obstetrical provider may choose to do it if bleeding has begun. Any reassuring signs should be mentioned to the mother. Bleeding during pregnancy is frightening, and speaking in a calm voice and providing a comfortable environment can allow the mother the opportunity to relax somewhat. The nurse places the woman on her side to ensure better blood flow once she arrives at the health care facility, and obtains baseline vital signs, particularly blood pressure and pulse rate. Questions to ask the mother include:

• How far along is the pregnancy?
• When did the bleeding begin?
• What color was the bleeding? Bright red indicates fresh, or ongoing bleeding.
• Was there pain with the bleeding?
• How many sanitary pads have you used since the bleeding started? This is to estimate amount of blood loss.
• Did you use anything to stop the blood flow, such as a tampon? Tampons will absorb the blood, and the true amount of bleeding may be masked.
• Is this the first episode of bleeding in this pregnancy? If not, obtain details about previous episodes.

The woman should be closely monitored for any signs of hemorrhage. The health of the fetus is monitored externally. Once bleeding has stopped and the mother must wait for the delivery, she can be at risk of perceiving the pregnancy as failed. This could result in her taking less care of herself, and thereby putting the fetus at risk. Continued reassurance for the mother helps prevent this from happening.

Prevention

Placenta previa is not preventable, as it is not possible to affect where the placenta will implant. However, once diagnosed, the mother may be instructed to avoid intercourse, get enough rest, and telephone the provider if any bleeding occurs.

KEY TERMS

Cervix— The cervix, or cervical os, is the opening between the vagina and the uterus. During labor the cervix thins and dilates, allowing the fetus to pass through, entering the birth canal and leaving through the vagina.

Cesarean birth— The terms cesarean section, birth, or delivery may be used interchangeably. This procedure to deliver a baby involves an abdominal incision made through the abdominal wall and into the uterus to extract the baby.

Hemorrhage— Hemorrhage refers to an excessive amount of blood lost within a very short time period. With massive blood loss the mother may have a rapid, weak pulse, drop in blood pressure, dizziness, pallor, clammy skin and appear disoriented. Hemorrhage is an emergency situation.

Placental abruption— This condition of pregnancy, also called abruptio placentae, is characterized by sharp pain, a hard, rigid abdomen and vaginal bleeding due to the detachment of the placenta from the uterine wall, placing the mother and fetus at great risk.

Resources

BOOKS

Creasy, Robert K. and Robert Resnik. Maternal-Fetal Medicine. Philadelphia: W.B. Saunders Company, 1999.

Feinbloom, Richard I. Pregnancy, Birth, and the Early Months. Cambridge, MA: Perseus Publishing, 2000.

Pillitteri, Adele. Maternal & Child Health Nursing. 3rd ed. Philadelphia: Lippincott, 1999.

PERIODICALS

Bekku, S., et al. "High Incidence of Respiratory Distress Syndrome (RDS) in Infants Born to Mothers with Placenta Previa." Journal of Maternal Fetal Medicine (March/April 2000): 110-113.

Wen, S. W., et al. "Placenta Previa and Male Sex at Birth: Results from a Population-based Study." Paediatric Perinatology and Epidemiology (October 2000): 300-304.

ORGANIZATIONS

American College of Obstetricians and Gynecologists. 409 12th St., S.W., PO Box 96920 Washington, D.C. 20090-6920. 〈http://www.acog.org〉.