Placenta Previa Health Article

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 lack 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.