Encyclopedia / P / Placenta Previa

Placenta Previa


A condition of pregnancy in which the placenta is implanted abnormally in the uterus so that it impinges on or covers the internal os (opening) of the uterine cervix.


Placenta previa is the most common cause of painless bleeding in the third trimester of pregnancy. There are usually four degrees of this condition: total placenta previa (the placenta covers the entire opening to the cervix); partial placenta previa (the placenta partially encroaches to within six centimeters of the cervical os); marginal placenta previa (the edge of the placenta is at the edge or margin of the opening of the cervix); and low-lying placenta or a low implantation (the placenta is low in the uterine segment and the edge of the placenta does not actually reach the cervical os but is very close to it).

Placenta previa occurs when the placenta – the spongy mass of blood vessels and tissue that forms within the uterus which supplies the baby with nutrition via the umbilical cord – forms unusually low in the uterus and covers part or all of the cervix.

Bleeding occurs because the lower third of the uterus stretches and thins somewhat during the last trimester, both to make room for the developing fetus and in preparation for birth. This stretching and thinning often causes a low-lying placenta to tear somewhat at its margins, causing bleeding.

If the placenta is lying partially or totally over the cervical opening, bleeding is virtually assured at some point. As every woman comes closer to term, the cervix begins to soften and change shape in preparation for the birth. It shortens (effaces) and begins to open up (dilates). Many women have some degree of these cervical changes weeks before labor begins.

These cervical movements create a problem for the placenta in a previa position. As the cervix starts to open up, it shears off the blood-filled placental attachments on top of it, causing the mother to bleed, sometimes dangerously. Nevertheless, less than one-half of one percent of women with previa suffer fatal complications, and nine out of ten babies survive.

There are serious problems associated with placenta previa. For the mother, repeated or severe bleeding can result in anemia and low blood volume, both of which can be countered through transfusions.

For the baby, the gravest danger after birth is respiratory distress syndrome due to premature delivery. With modern neonatal intensive care facilities and topflight medical care, babies born with respiratory distress do better now than ever before, but some babies do still die. Placenta previa can also cause the baby to be small for gestational age. This is called intrauterine growth retardation.

Babies of mothers with placenta previa seem to have a higher incidence of jaundice at birth (readily treated by exposure to special lights in the neonatal intensive care nursery). About 10 percent of all babies of mothers with placenta previa lose some blood, along with the mother, when bleeding occurs in the uterus. Sometimes the bleeding is severe enough that the baby needs a transfusion shortly after birth.


The cause of placenta previa is not known. It is more common in women who have already had children, older women and in women who smoke. A previous cesarean birth or induced abortion may increase the risk of placenta previa. A larger placenta increases the risk of placenta previa, because it is more likely for the edge of the placenta to lie near or over the cervical opening.


The first symptom is usually bright red, painless bleeding between weeks 28 and 38 of pregnancy. The initial episode of bleeding may or may not be followed by others.


Diagnosis is made in two major ways: a sudden onset of bleeding or discovery during an ultrasound (ordered for another reason).


If bleeding takes place, it is essential to obtain immediate care in a hospital Emergency Room.

There are two standard courses of treatment: immediate delivery by cesarean or "expectant management" – strict, in-hospital bedrest and frequent monitoring until the baby is mature enough to live without a respirator outside the uterus.

For a women with placenta previa, pelvic or rectal exams are never to be done unless she has been taken to a delivery room or operating room ready for an emergency C-section.


What can be done to prevent hemorrhaging?

Are there certain sleeping positions that will make hemorrhaging less likely?

If hemorrhaging begins, what should be done?

How serious is the risk to the unborn child's health?

Depending on the severity of the condition, how far along in the pregnancy is it safe to deliver the unborn child?

Is the unborn child getting enough nourishment if the placenta is separating from the uterine wall?

If a transfusion is necessary, how can you be sure that the blood is not contaminated?

What about sexual intercourse?

Will you have to monitor the baby? If so, how frequently?

Will an ultrasound be required?

Will the ultrasound hurt the unborn child?