If you have placenta previa, your placenta covers over the cervix, the outlet of your uterus. If your uterus is unusually shaped or its lining is scarred, or if your placenta is larger than normal, placenta previa might develop. There are several specific risk factors associated with a greater risk for placenta previa, including being more than 35 years old, smoking cigarettes, living at high altitude, carrying twins, being a mom several times over and having a history of placenta previa or surgery on your uterus.
Maternal Age
It is not clear why being older increases the risk that your placenta will attach low on your uterine wall and cover your cervix, but being an older mother does increase your risk for placenta previa. According to Dr. Steven Gabbe in "Obstetrics: Normal and Problem Pregnancies," if you are 35 years old, your risk for placenta previa is four times greater than for a younger woman. If you are 40 years or older, your risk is nine times higher. However the overall risk for placenta previa is fairly low, at four in 1,000 deliveries.
Smoking or High Altitude
If you smoke, among other pregnancy complications, you triple your risk of placenta previa, states "Obstetrics: Normal and Problem Pregnancies." Living at high altitude also increases your risk. The common factor is that lower levels of oxygen get to the placenta, which might increase its surface area to try to compensate. As the placenta spreads there is a greater risk that it will cover the cervix.
Twins
If you are carrying twins or more, you have increased risk for several pregnancy complications. "Obstetrics: Normal and Problem Pregnancies" reports that some studies have found an increased risk of placenta previa in twin pregnancies, while others have not. In a twin pregnancy, the placenta is larger and that could contribute to the increased risk of placenta previa.
Previous Pregnancies
If you have not had a baby before, you have a very low risk of placenta previa on your first pregnancy. The more children you have, the greater the risk of placenta previa in your next pregnancy. This might be due to changes in the shape of your uterus after several pregnancies. Although the mechanism is not clear, an abnormally shaped uterus is linked to a higher risk for placenta previa.
History
If you had placenta previa before, you are at eight times higher risk of having it again on a subsequent pregnancy, states "Obstetrics: Normal and Problem Pregnancies." If you have had uterine surgery, such as a cesarean delivery or a dilatation and curettage (D&C) to scrape your uterine lining, your risk increases because uterine surgery can scar the uterine lining. In trying to attach to unscarred areas, the placenta might implant low in the uterus, covering the cervix.
References
- "Obstetrics: Normal and Problem Pregnancies"; Steven Gabbe, Jennifer Niebyl and Joe Leigh Simpson; 2007
- MayoClinic.com: Placenta previa
