Placenta previa and vasa previa: Clinical sciences

Placenta previa is when placental tissue partially or completely covers the internal cervical os. When the placental edge is within 20 millimeters of the internal cervical os but isn’t actually covering it, it’s called a low-lying placenta. In contrast, in vasa previa, the fetal vessels either cross the internal os or are within 20 millimeters of it. The etiologies are unknown, though each condition has independent risk factors. These entities are important because they can cause bleeding, especially in labor or when membranes rupture. In placenta previa, the source of bleeding is maternal, whereas bleeding in vasa previa comes from the fetus.

Your first step in evaluating a patient presenting with a chief concern suggesting placenta previa or vasa previa is to do a CABCDE assessment to determine if they unstable. Unstable patients may have heavy vaginal bleeding, so prepare for urgent surgical management. Stabilize the airway, breathing, and circulation, and intubate the patient if necessary. Obtain IV access, and continuously monitor their vital signs. Initiate continuous fetal heart rate monitoring, and check for any signs of labor. Perform a sterile speculum exam to assess the volume of bleeding and check visually if the cervix is dilated.

Keep in mind that you should never perform a digital cervical exam on a patient with placenta previa or vasa previa, as it can disrupt the placenta and vessels and worsen the situation.

After the primary assessment, obtain a focused history, physical exam, and labs including CBC, PT, INR, PTT, fibrinogen, and a type and crossmatch. You may also need to perform an ultrasound to help with diagnosis, but don’t delay treatment while waiting for imaging.

Let’s talk about the history of those with placenta previa. Your patient may report dizziness, tunnel vision, and anxiety due to the acute blood loss, which occurs when shearing forces from uterine contractions and cervical changes disrupt the placental attachment site. There could also be a known abnormal placental location from an earlier ultrasound.

There are conditions that raise the risk of placenta previa, such as high parity, history of a prior c-section or other uterine surgery, advanced maternal age, multiple gestations, smoking, and in vitro fertilization.

A physical exam might reveal hypotension, tachycardia, and altered mental status. They may appear pale, and their skin might feel cold or clammy. Typically, there will be painless vaginal bleeding, which could spontaneously resolve or it might be ongoing. With severe bleeding, you may find fetal bradycardia as well.

As for the labs, they usually reveal anemia and possible thrombocytopenia, as well as elevated PT, INR, and PTT, and low fibrinogen as coagulation factors are consumed. Finally, an ultrasound will show the placenta either covering the cervical os or within 20 millimeters. This makes the diagnosis of placenta previa with hemorrhage and shock.

For treatment, start IV fluids and prepare to give blood transfusions, if needed. Deliver the patient by emergent c-section with immediate cord clamping, and give Rh immunoglobulin to Rh-negative patients to prevent rhesus alloimmunization in future pregnancies.

Now that we’ve talked about bleeding from a maternal source, let’s look at fetal blood loss from a vasa previa. In this case, bleeding occurs because the fetal blood vessels are unprotected and at risk for rupture and compression when the cervix dilates or the membranes rupture.

There could be evidence from an earlier ultrasound so consider risk factors such as velamentous cord insertion, meaning there are membranous umbilical vessels at the placental insertion site; marginal cord insertion, where the umbilical cord is inserted at or near the placental margin; succenturiate placenta, which is a placenta with one or more smaller accessory lobes; and in vitro fertilization.

On a physical exam, maternal vital signs are typically normal. Vaginal bleeding may persist or stop spontaneously. Assess fetal heart rate for a sinusoidal pattern that occurs with fetal anemia, or for sudden bradycardia with fetal decompensation. Expect normal labs, and if you have time for an ultrasound, it may show fetal vessels crossing or within 20 mm of the os. In this case, the diagnosis is lacerated vasa previa with fetal hemorrhage.

When it comes to treatment, perform an emergent c-section with immediate cord-clamping to prevent ongoing blood loss. Additionally, the neonate will often require immediate transfusion with type O-negative blood. Finally, provide maternal Rh immunoglobulin if they are Rh-negative.

Now that we’ve covered unstable patients, let’s move on to stable ones. Start with a focused history and physical exam. First, check if your patient has already had a second-trimester ultrasound, as this may have detected a placenta previa or vasa previa. On the other hand, they may present with painless vaginal bleeding.