eMedicine Specialties > Obstetrics and Gynecology > Obstetrical Complications
Placenta Previa
Updated: Aug 12, 2008
Introduction
Background
Placenta previa involves implantation of the placenta over the internal cervical os. Variants include complete implantation over the os (complete placenta previa), a placental edge partially covering the os (partial placenta previa) or the placenta approaching the border of the os (marginal placenta previa). A low-lying placenta implants in the caudad one half to one third of the uterus or within 2-3 cm from the os.
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Pathophysiology
Placental implantation is initiated by the embryo (embryonic plate) adhering in the lower (caudad) uterus. With placental attachment and growth, the developing placenta may cover the cervical os. However, it is thought that a defective decidual vascularization occurs over the cervix, possibly secondary to inflammatory or atrophic changes. As such, sections of the placenta having undergone atrophic changes could persist as a vasa previa.
A leading cause of third trimester hemorrhage, placenta previa presents classically as painless bleeding. Bleeding is thought to occur in association with the development of the lower uterine segment in the third trimester. Placental attachment is disrupted as this area gradually thins in preparation for the onset of labor. When this occurs, bleeding occurs at the implantation site as the uterus is unable to contract adequately and stop the flow of blood from the open vessels. Thrombin release from the bleeding sites promotes uterine contractions and a vicious cycle of bleeding-contractions-placental separation-bleeding.
Frequency
United States
Placenta previa occurs in 0.3-0.5% of all pregnancies. The risks increase 1.5- to 5-fold with a history of cesarean delivery. With an increased number of cesarean deliveries, this risk can be as great as 10%. Recent studies show that a previous cesarean delivery did not increase the odds of detecting a placenta previa on second-trimester ultrasonography. However, the rate of placental migration observed at 28-36 weeks' gestation may identify patients who are more likely to deliver vaginally with resolution of the previa.
Of all placenta previas, the frequency of complete placenta previa ranges from 20-45%, partial placenta previa accounts for approximately 30%, and marginal placenta previa accounts for the remaining 25-50%.
Mortality/Morbidity
Open table in new window
Table
| Morbidities | Relative Risk |
| Antepartum bleeding | 10 |
| Need for hysterectomy | 33 |
| Blood transfusion | 10 |
| Septicemia | 5.5 |
| Thrombophlebitis | 5 |
| Morbidities | Relative Risk |
| Antepartum bleeding | 10 |
| Need for hysterectomy | 33 |
| Blood transfusion | 10 |
| Septicemia | 5.5 |
| Thrombophlebitis | 5 |
The perinatal mortality rate associated with placenta previa ranges from 2-3%.
Maternal mortality is 0.03% in the United States.
Race
Placenta previa has no predilection for any race.
Sex
Placenta previa only occurs in pregnant women.
Age
Age is associated with a varying prevalence of placenta previa. The risk of placenta previa in relation to age is as follows:
- Aged 12-19 years - 1%
- Aged 20-29 years - 0.33%
- Aged 30-39 years - 1%
- Older than 40 years - 2%
Clinical
History
The classic presentation of placenta previa is painless vaginal bleeding.
- Nearly two thirds of symptomatic patients present before 36 weeks' gestation, with half of these patients presenting before 30 weeks' gestation.
- This hemorrhage often stops spontaneously and then recurs with labor.
Physical
- Any pregnant patient beyond the first trimester who presents with vaginal bleeding requires a speculum examination followed by diagnostic ultrasound, unless previous documentation confirms no placenta previa.
- Because of the risk of provoking life-threatening hemorrhage, a digital examination is absolutely contraindicated until placenta previa is excluded.
- Uterine activity monitoring reveals that approximately 20% of patients have concurrent contractions with their bleeding.
Causes
- Hemorrhaging, if associated with labor, would be secondary to cervical dilatation and disruption of the placental implantation from the cervix and lower uterine segment. The lower uterine segment is inefficient in contracting and thus cannot constrict vessels as in the uterine corpus, resulting in continued bleeding.
- Advancing age (>35)
- Multiparity
- Infertility treatment
- Multiple gestation (larger surface area of the placenta)
- Erythroblastosis
- Prior uterine surgery
- Recurrent abortions
- Nonwhite ethnicity
- Low socioeconomic status
- Short interpregnancy interval
- Smoking
- Cocaine use
- Other causes include digital exam, abruption (pre-eclampsia, chronic hypertension, cocaine use, etc) and other causes of trauma (eg, postcoital trauma).
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| References |
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References
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Further Reading
Keywords
placenta previa, complete placenta previa, partial placenta previa, marginal placenta previa, low-lying placenta, placenta accreta, placenta increta, placenta percreta, cesarean delivery, cesarean hysterectomy, transvaginal sonography, transvaginal ultrasonography, tocolysis, continuous fetal monitoring, accreta, increta, percreta, transvaginal ultrasound
