Michael A. Belfort
Alireza A. Shamshirsaz
Placenta previa is defined as placental tissue covering the internal cervical os (Figure 4.8.1). The pathophysiology of exactly why this occurs is not clear.
The prevalence of placenta previa is estimated to be 4 in 1,000 live births (1).
Several risk factors are associated with placenta previa including the following:
Previous placenta previa
Previous cesarean delivery
Other independent risk factors are advanced maternal age and high parity, use of assisted reproduction technologies, previous invasive uterine procedures, and smoking.
Placenta previa should be considered for all pregnant women presenting with vaginal bleeding after 20 weeks of gestation.
Digital cervical examination should be avoided until placenta previa is ruled out.
Cautious sterile speculum vaginal examination may be performed to evaluate the amount of bleeding with specific caution not to cause further bleeding by disturbing any placenta that may be covering the internal cervical os. Gentle transabdominal ultrasound examination may be helpful as a first step if available and can give important information that may allow avoidance of pelvic examination. In those cases where transabdominal ultrasound is not helpful, gentle transvaginal ultrasound examination, avoiding placement of the probe too forcefully or deeply into the vaginal canal or the cervical os, may confirm the diagnosis.
Fetal evaluation can be performed by asking about fetal movements and by monitoring the fetal heart rate tracing.
Definitive diagnosis is generally made via imaging (see the section for imaging and other diagnostics).
It is essential to obtain accurate obstetrical and surgical history as placenta previa significantly increases the risk of placenta accreta spectrum in cases of prior cesarean delivery or uterine surgeries (4).
Placenta previa should be considered in any case of vaginal bleeding in pregnancy. The most common differentials are the following:
Labor, bloody show
Vaginal, cervical, or uterine pathology or injury
Before 20 weeks of gestation, cervical insufficiency and early pregnancy loss should be considered.
Serial ultrasound monitoring of placental location before delivery as described in the section for imaging
Vaginal bleeding should be monitored, and precautions should be given to the patient regarding pelvic rest (specifically no sexual intercourse or vaginal penetration).
IMAGING AND OTHER DIAGNOSTICS
Universal screening for placental location during the midtrimester anatomy scan
Diagnosis is usually made by identifying placental tissue covering the internal cervical os with the use of ultrasound in the second or third trimester (including careful transvaginal ultrasound) (Figures 4.8.2 and 4.8.3). The distance of the placental edge from the internal os should be documented if placental tissue is not actually impinging or covering the os.
If the placenta is not covering the internal cervical os, but the lower placental edge is <2 cm from the os, a diagnosis of “low-lying placenta” is made (Figure 4.8.4).
Several views of the placenta should be obtained in multiple planes, including the sagittal and transverse planes, with a full, partially full, and empty bladder if necessary to confirm the diagnosis.
Up to 55% of placenta previa diagnosed at 20 to 23 weeks of gestation will resolve by the time of delivery if the patient has no history of cesarean delivery (5).
If a diagnosis of placenta previa or low-lying placenta is made in the second trimester, follow-up ultrasound should be performed at 32 weeks to reevaluate placental location. If there is persistent previa, a repeat follow-up ultrasound at 36 weeks is warranted (6).
Specific evaluation of the cord insertion and fetal vessels is important because placenta previa increases the risk of velamentous cord insertion and vasa previa (especially when there is a low-lying placenta).
Diagnosis may be challenging with advancing gestational age because of fetal parts obscuring the placental views.
Always evaluate the placenta and uteroplacental interface for signs of placenta accreta spectrum when making a diagnosis of low-lying placenta or placenta previa.
Magnetic resonance imaging (MRI) should be reserved for cases of complicated placenta previa including suspicion for placenta accreta spectrum.
If placenta previa persists up to 36 weeks of gestation in an asymptomatic patient, the American College of Obstetricians and Gynecologists (7) recommends a scheduled prelabor cesarean delivery at 36+0 to 37+6 weeks.
In those cases where there is preterm bleeding, individualized planning is recommended depending on the patient’s hemodynamic stability, amount of bleeding, fetal status, and other risk factors. The presence of persistent/active labor, nonreassuring fetal heart status unresponsive to resuscitation, persistent bleeding causing maternal hemodynamic instability, should compel expeditious delivery. In cases where there is minor bleeding not resulting in any of the aforementioned complications, conservative management may be considered until the scheduled delivery. Any significant bleeding after 34 weeks of gestation, however, should prompt delivery (8).
If a low-lying placenta persists to 36 weeks of gestation, a trial of labor can be discussed, considering the distance of the placental edge from the cervical os. The closer the placental edge is to the internal cervical os, the greater the risk of significant bleeding (6).
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