placenta previa

Placenta Previa 

Definition

Placenta previa is a condition that occurs during pregnancy when the placenta is abnormally placed, and partially or totally covers the cervix.

Description

The uterus is the muscular organ that contains the developing baby during pregnancy. The lowest segment of the uterus is a narrowed portion called the cervix. This cervix has an opening (the os) that leads into the vagina, or birth canal. The placenta is the organ that attaches to the wall of the uterus during pregnancy. The placenta allows nutrients and oxygen from the mother's blood circulation to pass into the developing baby (the fetus) via the umbilical cord.

During labor, the muscles of the uterus contract repeatedly. This allows the cervix to begin to grow thinner (called effacement) and more open (dilatation). Eventually, the cervix will become completely effaced and dilated, and the baby can leave the uterus and enter the birth canal. Under normal circumstances, the baby will emerge through the mother's vagina during birth.

In placenta previa, the placenta develops in an abnormal location. Normally, the placenta should develop relatively high up in the uterus, on the front or back wall. In about one in 200 births, the placenta will be located low in the uterus, partially or totally covering the os. This causes particular problems in late pregnancy, when the lower part of the uterus begins to take on a new formation in preparation for delivery. As the cervix begins to efface and dilate, the attachments of the placenta to the uterus are damaged, resulting in bleeding.

Causes and symptoms

While the actual cause of placenta previa is unknown, certain factors increase the risk of a woman developing the condition. These factors include:

• having abnormalities of the uterus
• being older in age
• having had other babies
• having a prior delivery by cesarean section
• smoking cigarettes

When a pregnancy involves more than one baby (twins, triplets, etc.), the placenta will be considerably larger than for a single pregnancy. This also increases the chance of placenta previa.

Placenta previa may cause a number of problems. It is thought to be responsible for about 5% of all miscarriages. It frequently causes very light bleeding (spotting) early in pregnancy. Sometime after 28 weeks of pregnancy (most pregnancies last about 40 weeks), placenta previa can cause episodes of significant bleeding. Usually, the bleeding occurs suddenly and is bright red. The woman rarely experiences any accompanying pain, although about 10% of the time the placenta may begin separating from the uterine wall (called abruptio placentae), resulting in pain. The bleeding usually stops on its own. About 25% of such patients will go into labor within the next several days. Sometimes, placenta previa does not cause bleeding until labor has already begun.

Placenta previa puts both the mother and the fetus at high risk. The mother is at risk of severe and uncontrollable bleeding (hemorrhage), with dangerous blood loss. If the mother's bleeding is quite severe, this puts the fetus at risk of becoming oxygen deprived. The fetus' only source of oxygen is the mother's blood. The mother's blood loss, coupled with certain changes that take place in response to that blood loss, decreases the amount of blood going to the placenta, and ultimately to the fetus. Furthermore, placenta previa increases the risk of preterm labor, and the possibility that the baby will be delivered prematurely.

Diagnosis

Diagnosis of placenta previa is suspected whenever bright red, painless vaginal bleeding occurs during the course of a pregnancy. The diagnosis can be confirmed by performing an ultrasound examination. This will allow the location of the placenta to be evaluated.

While many conditions during pregnancy require a pelvic examination, in which the health care provider's fingers are inserted into the patient's vagina, such an examination should never be performed if there is any suspicion of placenta previa. Such an examination can disturb the already susceptible placenta, resulting in hemorrhage.

Sometimes placenta previa is found early in a pregnancy, during an ultrasound examination performed for another reason. In these cases, it is wise to have a repeat ultrasound performed later in pregnancy (during the last third of the pregnancy, called the third trimester). A large percentage of these women will have a low-lying placenta, but not a true placenta previa where some or all of the os is covered.

Treatment

Treatment depends on how far along in the pregnancy the bleeding occurs. When the pregnancy is less than 36 weeks along, the fetus is not sufficiently developed to allow delivery without a high risk of complications. Therefore, a woman with placenta previa is treated with bed rest, blood transfusions as necessary, and medications to prevent labor. After 36 weeks, the baby can be delivered via cesarean section. This is almost always the preferred method of delivery in order to avoid further bleeding from the low-lying placenta.

Prognosis

In cases of placenta previa, the prognosis for the mother is very good. However, there is a 15-20% chance the infant will not survive. This is 10 times the death rate associated with normal pregnancies. About 60% of these deaths occur because the baby delivered was too premature to survive.

Prevention

There are no known ways to insure the appropriate placement of the placenta in the uterus. However, careful treatment of the problem can result in the best chance for a good outcome for both mother and baby.

Resources

Organizations

American College of Obstetricians and Gynecologists. 409 12th Street, S.W., P.O. Box 96920, Wasington, DC 20090-6920.

Key terms

Cesarean section — Delivery of a baby through an incision in the mother's abdomen instead of through the vagina.

Labor — The process during which the uterus contracts, and the cervix opens to allow the passage of a baby into the vagina.

Placenta — The organ that provides oxygen and nutrition from the mother to the baby during pregnancy. The placenta is attached to the wall of the uterus and leads to the baby via the umbilical cord.

Umbilical cord — The blood vessels that allow the developing baby to receive nutrition and oxygen from its mother; the blood vessels also eliminate the baby's waste products. One end of the umbilical cord is attached to the placenta and the other end is attached to the baby's belly button (umbilicus).

Vagina — The birth canal; the passage from the cervix of the uterus to the opening leading outside of a woman's body.

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placenta /pla·cen·ta/ (plah-sen´tah) pl. placentas, placen´tae   [L.] an organ characteristic of true mammals during pregnancy, joining mother and fetus, providing endocrine secretion and selective exchange of soluble bloodborne substances through apposition of uterine and trophoblastic vascularized parts.placen´tal

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placenta accre´ta  one abnormally adherent to the myometrium, with partial or complete absence of the decidua basalis.

circumvallate placenta  one in which a dense peripheral ring is raised from the surface and the attached membranes are doubled back over the placental edge.

deciduate placenta , deciduous placenta a placenta or type of placentation in which the decidua or maternal parts of the placenta separate from the uterus and are cast off together with the trophoblastic parts.

fetal placenta  the part of the placenta derived from the chorionic sac that encloses the embryo, consisting of a chorionic plate and villi.

hemochorial placenta  one in which maternal blood comes in direct contact with the chorion, as in humans.

placenta incre´ta  placenta accreta with penetration of the myometrium.

maternal placenta  the maternally contributed part of the placenta, derived from the decidua basalis.

placenta membrana´cea  one that is abnormally thin and spread out over an unusually large area of the uterine wall.

placenta percre´ta  placenta accreta with invasion of the myometrium to its peritoneal covering, sometimes causing rupture of the uterus.

placenta pre´via  one located in the lower uterine segment, so that it partially or completely covers or adjoins the internal os.

placenta spu´ria  an accessory portion having no blood vessel attachment to the main placenta.

placenta succenturia´ta , succenturiate placenta an accessory portion attached to the main placenta by an artery or vein.

villous placenta  one characterized by the presence of villi that are outgrowths of the chorion.

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placenta pre·vi·a (prv-)

n.

A condition in which the placenta is implanted in the lower segment of the uterus so that it is adjacent to or obstructs the internal opening of the cervix. It may cause maternal hemorrhage prior to or during labor. Also called placental presentation.

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placenta previa

[prē′vē·ə]

a condition of pregnancy in which the placenta is implanted abnormally in the uterus so that it impinges on or covers the internal os of the uterine cervix. It is the most common cause of painless bleeding in the third trimester of pregnancy. Its cause is unknown. The incidence of the condition increases with increased parity from approximately 1 in 1500 primiparas to approximately 1 in 20 grand multiparas. Even slight dilation of the internal os can cause enough local separation of an abnormally implanted placenta to result in bleeding. If severe hemorrhage occurs, immediate cesarean section is usually required to stop the bleeding and to save the mother's life; it is performed regardless of the stage of fetal maturity. Before hemorrhage, placenta previa may be diagnosed by ultrasonography and treated with complete bed rest under close observation. Even at rest sudden massive hemorrhage can occur without warning. Vaginal examination is usually contraindicated if placenta previa is present or suspected because palpation can cause local placental separation and precipitate hemorrhage. Cautious and very gentle intracervical palpation may be performed to determine the existence and exact extent of placenta previa. Before this examination an IV infusion is begun, the woman's blood is typed and crossmatched, and preparations for immediate cesarean section are made. If the placenta is next to or near, rather than touching or covering, the cervical os, labor and vaginal delivery may be attempted. Central placenta previa refers to a placenta that has grown to cover the internal cervical os completely; low-lying placenta identifies a placenta that is just within the lower uterine segment; and partial or marginal placenta previa is a condition in which the placenta partially covers the internal cervical os. Also called

Usage notes: (informal)

previa, placental presentation. Compare abruptio placentae.

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placenta (plsen´t),

n the organ of metabolic interchange between the fetus and the mother.

placenta abruptio

n a condition in which a typically positioned placenta detaches from the uterine wall prior to delivery, which may threaten both the viability of baby and the life of the mother.

placenta previa

n atypical placental positioning and attachment within the inferior third of uterus, which may cover the cervix in part or fully.

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placenta previa

Obstetrics A condition in which the placenta implants in the lower uterus and obstructs the birth canal Etiology Scarred endometrium, a large placenta, abnormal placentation Incidence ±1 in 200 births; 1 in 20 with multiparas, doubled in multiparas Risk factors Multiparity, multiple pregnancy, prior C-section if scar is low and close to the cervix region