The placenta provides the vital physical connection between a pregnant woman and the fetus. It delivers nutrients and oxygen from the mother's circulation and allows removal of waste products from the fetus. Placenta previa is a rare but potentially life-threatening condition that occurs when the placenta implants too low on the wall of the uterus, so that it impinges on the uterine opening (cervix).
Placenta
Shortly after implantation in the uterus, the outer layer of cells of the embryo, called the trophoblast, form the fetal layer of the placenta. On the maternal side, implantation triggers the stromal cells of the uterus to undergo a change called the decidual reaction that creates the decidua, a layer of tightly adhering cells that surrounds the embryo. The decidua eventually fuses with the embryonic layer to form the mature placenta. The placenta attaches to the wall of the uterus, and the umbilical cord runs from the placenta into the fetus.
Placenta previa
In placenta previa, the placenta forms low on the uterine wall so that it covers the cervix, the narrow outlet at the bottom of the uterus. Doctors classify placenta previa according to how much of the cervix is covered. In marginal placenta previa, the placenta lies unusually close to the cervix. In partial and total placenta previa, the placenta blocks part or all of the cervix, respectively. Placenta previa of any type is a rare complication, occurring in 1 in 4000 deliveries, according to educator and physician Dr. Steven Gabbe in the book "Obstetrics: Normal and Problem Pregnancies."
Risk Factors
Gabbe outlines several factors that can increase the risk of placenta previa. Decreased oxygen delivery to the fetus, from maternal smoking or living at a high altitude, increases risk because the placenta spreads to maximize the surface area available for oxygen diffusion. Changes to the uterus from previous pregnancies or surgeries can cause the placenta to attach low and block the cervix. For unclear reasons, mothers older than 40 years of age have nine times the risk of placenta previa.
Complications
As the delivery date approaches, the cervix begins to open (dilate). If the placenta lies over the cervix, dilation disrupts the placenta, which can cause massive maternal bleeding leading to shock or even death. The mother might need an emergency C-section to deliver the baby and stop the bleeding. In such cases, the baby might be born prematurely.
Diagnosis
Bright red vaginal bleeding in the second trimester can indicate placenta previa.
Most cases of placenta previa are discovered by ultrasound before the mother or baby faces serious risks, according to the Mayo Clinic.
Treatment
Treatment options depend on the severity of bleeding and can range from bed rest at home to hospitalization with blood transfusions and medications to inhibit premature labor, according to the Mayo Clinic. If the placenta covers the cervix at all, the doctor will schedule a C-section as soon as the baby can be safely delivered (usually 36 weeks).
References
- Mayo Clinic: Placenta Previa
- "Obstetrics: Normal and Problem Pregnancies"; Steven Gabbe, Jennifer Niebyl and Joe Leigh Simpson; 2007
