Placenta previa is an uncommon pregnancy complication wherein the placenta implants over the cervical os (opening to the cervix), blocking the cervix. In a normal pregnancy, the placenta is usually planted high up in the uterus. Implantation can completely cover the os (total placenta previa), the placental edge can partially cover the os (partial placenta previa) or the placenta can come close to the border of the os (marginal placenta previa). It is usually seen when a pregnant woman before 36 weeks gestation has painless vaginal bleeding. This can cause a problem before or after birth to both mother and the baby. According to the American Pregnancy Association, placenta previa occurs in one in 200 pregnancies in the third trimester, the risk increases in women with a history of cesarean delivery or any other uterine surgery.
Smoking and Cocaine Use
Smoking and cocaine use can cause the blood vessels supplying the placenta to be constricted, limiting its function and making it unable to supply oxygen and nutrients to the fetus. It is this abnormal vascular blood supply to the endometrium that is thought to cause placenta previa. Smoking deprives the placenta of oxygen, so the placenta is forced to spread out to gain more surface area.
Maternal Complications
Multiparity (multiple previous births), abortions, previous cesareans, are also risk factors for placenta previa. These maternal complications affect the womb, the uterus wall can easily undergo scarring and damage, making it susceptible to improper implantation. The damage done makes decreases viable womb space to allow the placenta to implant properly.
A large placenta can occur when the woman has a history of twins, triplets or multiple births, or if a mother has a previous history of antibody blood group incompatibility with her fetus. A large placenta causes the placenta to stretch out further than normal, increases its risk of covering the cervical opening.
Diagnosis and Other Differentials
Even with these risk factors, the risk for developing placenta previa is still relatively low. It is important to differentiate placenta previa from other pregnancy complications such as abruption placentae, inflammation of the cervix, preterm labor, inflammation of the vagina, tearing of the cervix, or even a miscarriage (spontaneous abortion). Any second or third trimester sonogram performed for any reason should discover placenta previa if it is present.
