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| Placenta Previa By: Mandy Hobby |
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| Image reproduced with permission from thefetus.net |
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ABSTRACT
Placenta previa is a condition in which the placenta is located low in the uterine cavity, partially or completely covering the internal os (opening of the cervix). Previa is the number one cause of painless, vaginal bleeding in the second and third trimesters. A cesarean section is usually performed on mothers who have placenta previa. This article discusses the causes and effects of previa, as well as the different types of placenta previa. The role of ultrasound in diagnosing placenta previa is also discussed.
KEYWORDS
Placenta previa, complete previa, partial previa, low-lying previa, marginal previa
INTRODUCTION
Placenta previa is a condition in which the placenta is located low in the uterine cavity, partially or completely covering the internal os (opening of the cervix). This can cause bleeding and interfere with a normal vaginal delivery of a newborn. Placenta previa can cause serious mortality and morbidity to both the mother and the fetus. It occurs in about 1 in every 200 pregnancies.1
BODY
The placenta is made of tissues from both the mother and fetus. It attaches to the uterine wall serves as the primary site of nutrient and gas exchange between the maternal and fetal tissues. However, there are times when the placenta may implant in the lower portion of the uterus near the cervix. This is called placenta previa and may cause problems for both the mother and fetus if this should occur.
There are three types of placenta previa: complete, in which the cervix is completely covered. Partial, only a portion of the internal os is covered. Low-lying or marginal previa is when the placenta is close but does not interfere with the opening of the cervix. A normal delivery cannot be attempted with any type of previa because it can cause severe hemorrhage and even lead to maternal death.2 A cesarean section is usually performed on mothers who have placenta previa. |
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| Image reproduced with permission from thefetus.net |
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A major symptom of previa is painless vaginal bleeding. This may begin as early as the 24th to 26th week, though more common during the last 4 or 5 weeks of pregnancy.2 When the cervix begins to thin out and the placenta is covering the cervical opening, blood vessels that attach to the placenta rupture causing bleeding. Portions of the placenta may be torn from their attachments of the uterine wall, leading to bleeding ranging from light to profuse. The blood is usually a bright red, indicating that the bleeding is fresh. If the bleeding cannot be controlled an immediate cesarean section is usually done, regardless of the length of the pregnancy.3 Some marginal previas can be delivered without cesarean surgery.3 Other symptoms may include: spotting during the first and second trimesters and uterine cramping during the onset of bleeding.4
When previa is suspected, a vaginal examination is usually avoided because it may trigger heavy bleeding. Some women who have not experienced vaginal bleeding learn during a routine ultrasound examination that they have a low-lying placenta. A pregnant woman should not be too worried if this happens to her, especially if she is in the first half of the pregnancy. More than 90 percent of the time, as pregnancy progresses the placenta may migrate farther from the cervix, growing toward the rich blood supply near the fundus, and may no longer be a problem.3 Prognosis for a woman diagnosed with previa is usually excellent, if the condition has been managed appropriately.
Treatment for previa may be classified as either delayed or active. A woman who has been diagnosed with previa may need to stay in the hospital until delivery. If the bleeding stops, her physician will continue to monitor her and the baby (this is delayed treatment). Active treatment is for when the bleeding does not stop, or if the woman goes into labor, her physician will recommend an immediate c-section. A cesarean delivery is usually recommended for nearly all women with placenta previa because c-sections usually can prevent severe bleeding.3
The actual cause of placenta previa is unknown. Possible causes include a scarred endometrium, a large placenta, an abnormal uterus, or abnormal formation of the placenta. Risk factors associated with placenta previa are increased parity, prior cesarean section, prior uterine surgery, advanced maternal age, and previous abortion. Women who have had placenta previa in a previous pregnancy have a 4 to 8 percent chance of recurrence.3 Some experts believe that placental implantation cannot take place on the same part of the uterine wall more than once.
If a woman has had several pregnancies, the lower part of the uterus may be the only place left on the uterine wall for the placenta to implant. This would explain why previa has been linked to women who are multiparity. The incidence increases with each pregnancy, and it is estimated that 1 in 20 women who have had 6 or more previous deliveries are at risk.4 The exact reason as to why prior c-section is a risk factor is unknown but has been linked to uterine scarring. Smoking during pregnancy has also been attributed to the cause of previa, although studies show it to be a weak association.5 |
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| Image reproduced with permission from thefetus.net |
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Placenta previa can affect both the mother and fetus in a number of ways. Maternal complications include: major hemorrhage, shock, and death. The risk of infection and developing blood clots also increases. There is an increased risk of heavy bleeding not only during pregnancy, but during and after delivery as well. A blood transfusion may be required due to a major loss of blood. Pitocin is usually given after birth to make the uterus contract and stop bleeding, with previa the uterus may not contract as well due to the location of the placenta and the bleeding may not be controlled properly.6 There is a 1 in 10 chance of developing placenta accreta with future pregnancies. Placenta accreta is when the placenta implants low in the uterus and does not separate easily after delivery of the baby.
Complications are also a concern for the fetus as well. These include: slowed fetal growth, preterm birth, birth defects, respiratory distress syndrome, and fetal death. Prematurity is responsible for about 60 percent of infant deaths in cases of placenta previa.6 Fetal blood loss or hemorrhage may occur due to the separation of the placenta from the uterine wall during labor, as well as, surgical entry during a c-section. Congenital anomalies include: cardiovascular, gastrointestinal, respiratory, genitourinary, dermatologic, musculoskeletal, neurologic, and chromosomal anomalies.7 Anemia has also been found to be a fetal complication of previa. Corticosteroid drugs are given if the mother is to deliver before 34 weeks to aid in the development of the fetal lungs.7
CONCLUSION
In conclusion, women who have a history of prior c-section, abortion/miscarrage, or uterine surgery are at greater risk for developing placenta previa. Advanced maternal age and increased parity are also risk factors. The actual cause of previa may be unknown, but a scarred endometrium, abnormal uterus, or large placenta are found to be possible causes. There are a number of effects that placenta previa has on both the mother and the fetus, both including death. If vaginal bleeding should occur at any point during pregnancy, the woman should call her physician for further care. With the proper diagnosis and treatment of previa, the prognosis is good.
WORKS CITED
1. Laughon, MS, MD, S. Katherine. “Prior Cesarean and risk for placenta previa on 2nd trimester ultrasound.” American College of Obstetrics and Gynecology. [online] 2005. Vol.105. pgs. 962-965. http://www.greenjournal.org/cgi/content/full/105/5/962.
2. Ko, MD, Patrick. “Placenta Previa.” [online] January 6, 2005. http://www.emedicine.com/emerg/topic427.htm
3. “Placenta Previa.” [online] 2006. http://www.pregnancy.about.com/cs/placentaprevia/a/previa.htm
4. “Placenta Previa.” Medlineplus Encyclopedia. April 13, 2006. [online]
http://www.hlm.nih.gov/mededlineplus/ency/article/000900.htm
5. Anath, Cande V. “Maternal Cigarrette smoking as risk for placenta previa.” American Journal of Epidemology. 1996 [online] vol. 44, No.9. Pgs. 881-889
http://www.aje.oxfordjournals.org/cgi/content/abstract/144/a/881
6. Haratz-Rubenstein, MD, Natan. “Placenta Previa.” May 2005. [online]
http://www.babycenter.com/refcap/830.html
7. Crane, MD, Joan M. “Neonatal outcomes with placenta previa.” American College of Obstetrics and Gynecology. 1999. [online] Vol.93. pgs-541-544.
http://ww.greenjouranl.org/cgi/content/full/93/4/541
*Images reproduced with permission from thefetus.net
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