The placenta should be clearly visible by the beginning of 2nd trimester. Here are the characteristics of a normal placenta:
- Uniform echogenicity (shade of gray) and rounded margins
- Slight hypoechoic area behind the placenta
- Usually, the cord will be centrally inserted
- Average thickness of 2-4 cm.
To locate the placenta follow these steps:
- Starting at the superior end of the uterus, sweep from left to right in a sagittal plane.
- Follow the same pattern for more inferior sections of the uterus
- Identify the margins of the placenta.
- If the fetus is creating artifact and obscuring a posterior placenta, scan laterally.
Placenta previa refers to when the placenta is covering the internal os. This will commonly present as painless vaginal bleeding.
The earlier that placenta previa is detected, the less likely it is to be present at delivery since the placenta appears to migrate during pregnancy. However, if the placenta is covering the internal os at the onset of labor, a C-section is indicated.
Transvaginal ultrasound is preferred over transabdominal ultrasound for detecting low lying placenta and placenta previa.
Marginal placenta previa is when the placenta is extending to the edge of the internal os.
A low lying placenta is when the placenta is within 2 cm of the internal os.
Placenta Accreta Spectrum
Placenta accreta spectrum, also known as morbidly adherent placenta or just placenta accreta, is the excessive proliferation of the placenta into the myometrium. It is defined along a spectrum based on how deep into the myometrium the placenta invades (see figure below).
- Risk factors: Prior uterine surgery, placenta previa, assisted reproduction
- Placenta Accreta: Placenta is in contact with the myometrium
- Placenta Increta: Placenta invades the myometrium
- Placenta Percreta: Placenta invades past the myometrium into adjacent structures
Placenta accreta spectrum has these features on ultrasound:
- Discontinuous myometrium or thin (<1 mm) myometrium
- Absent retroplacental hypoechoic space that is seen in a normal placenta
- Increased retroplacental vascularity
- Placental lacunae – hypoechoic areas within the placenta
- This is said to look like Swiss cheese
- These will have turbulent blood flow on color doppler
- In placenta percreta: Abnormal or loss of interface between uterus and adjacent structures (e.g. bladder) with chaotic vascularity
Placental abruption occurs when the placenta prematurely separates from the uterine wall. For the fetus, this can lead to preterm birth or stillbirth. For the mother, abruption can lead to large blood loss, DIC, or even death.
Diagnosing placental abruption with ultrasound can be difficult due to the varying presentation and severity. The clinical and ultrasound appearance of abruption depends on several factors:
- The location of the placenta
- The location of the separation and bleeding (pre-placental, retroplacental, placental margin)
- How old the bleeding is
How do these factors influence ultrasound findings? Acute bleeding will appear as hypoechoic or isoechoic to the placenta. The blood will appear hyperechoic after 1-2 weeks. Therefore, a blood clot over the internal os can look similar to a placenta previa. Additionally, a retroplacental abruption can mimic the appearance of the hypoechoic space behind a normal placenta.
With those factors in mind, here are some ultrasound findings that may suggest the presence of an abruption:
- The placenta “jiggles” with quick transducer pressure. This is known as the jello sign.
- Retroplacental, preplacental, or placental margin collection of fluid — will have a lack of flow.
- Placental heterogeneity
- Varying placental thickness