Diagnostic Keys for Placenta and Umbilical Cord Imaging

The placenta and umbilical cord play crucial roles in fetal development, serving as the maternal-fetal interface. Therefore, precise imaging of the placenta is critical to detect and manage potential pregnancy complications early. These vital structures support maternal-fetal oxygen exchange, nutrient delivery, and waste removal. Consequently, any disruption to these structures can seriously compromise fetal health and maternal safety.

Evaluating Placenta Previa with Ultrasound

Initially, clinicians use ultrasound as the first-line modality for evaluating placental position. For instance, transabdominal ultrasound offers a rapid screening assessment of the relationship between the placental edge and the internal cervical os. However, transvaginal ultrasound remains the gold standard for diagnosing placenta previa. Specifically, this technique provides superior resolution and avoids acoustic shadowing from the fetal presenting part.

Moreover, we define placenta previa when the placental tissue covers the internal cervical os. Consequently, if the placenta lies within twenty millimeters of the os, we classify it as low-lying. Historically, radiologists used terms like partial or marginal previa, but modern guidelines have retired these classifications.

Therefore, we schedule a repeat scan at 32 weeks of gestation if we detect previa during the second trimester. Fortunately, more than ninety percent of early low-lying placentas resolve before term due to lower uterine segment expansion.

Advanced Imaging of the Placenta in Accreta Spectrum

Indeed, placenta accreta spectrum (PAS) is a severe obstetric condition where the placenta abnormally adheres to the myometrium. Therefore, precise identification of PAS is crucial to reduce maternal morbidity and hemorrhage risk. Initially, ultrasound acts as the primary tool, but magnetic resonance imaging provides valuable adjunctive information. Specifically, magnetic resonance imaging is helpful when ultrasound results are equivocal or when the placenta is posterior.

Furthermore, the joint consensus statement from abdominal and urogenital radiology societies outlines key findings for imaging of the placenta using MRI. For example, dark T2-weighted intraplacental bands represent a primary indicator of invasion. Additionally, other signs include uterine bulging, loss of the retroplacental T2-dark line, and abnormal vasculature in the placental bed.

Consequently, identifying these features allows multidisciplinary teams to plan surgeries effectively. For instance, a delivery in a tertiary care center reduces surgical risks. Thus, proper imaging of the placenta directly improves maternal and neonatal survival rates.

Vasa Previa and Cord Insertion Abnormalities

In addition to placental position, we must evaluate the umbilical cord and its insertion site. Specifically, vasa previa occurs when unprotected fetal blood vessels run through the amniotic membranes over the cervix. Therefore, any rupture of membranes can cause catastrophic fetal exsanguination. For this reason, early detection of vasa previa is absolutely essential.

Fortunately, we can reliably diagnose this condition during the mid-trimester fetal anatomy scan. To achieve this, radiologists systematically evaluate the lower uterine segment using transvaginal ultrasound with color Doppler. Moreover, documenting the placental cord insertion site helps identify at-risk pregnancies.

For instance, a velamentous cord insertion, where the cord inserts into the membranes, strongly associates with vasa previa. Additionally, succenturiate placental lobes or multiple gestations increase the risk. Consequently, finding these anomalies should prompt a targeted search for aberrant fetal vessels.

Finally, when we confirm vasa previa, clinical management changes significantly. Specifically, obstetricians typically plan a scheduled cesarean delivery between 34 and 37 weeks. Additionally, they may recommend antenatal corticosteroids and hospitalization to optimize fetal outcomes.

Frequently Asked Questions

Q1: Why is transvaginal ultrasound preferred over transabdominal ultrasound for diagnosing placenta previa?

Transvaginal ultrasound offers significantly higher resolution and is not hindered by acoustic shadowing from the fetal presenting part, making it the gold standard for defining the relationship between the placental edge and the internal cervical os.

Q2: When should radiologists recommend magnetic resonance imaging (MRI) for placental evaluation?

MRI is highly recommended as an adjunctive tool when ultrasound results are equivocal, when a suspected placenta accreta spectrum (PAS) involves a posterior placenta, or when clinicians need to assess the exact depth and topography of myometrial invasion before surgery.

Q3: What are the primary risk factors for vasa previa?

The primary risk factors include a velamentous cord insertion, succenturiate or bilobed placentas, multiple gestations, a history of second-trimester low-lying placenta or placenta previa, and pregnancies achieved through in vitro fertilization.

References

1. Leal PV et al. Imaging of the Placenta and Umbilical Cord: What Radiologists in Common Practice Need to Know. Radiographics. 2026 Jun undefined. doi: 10.1148/rg.250044. PMID: 42207685.
2. Jha P, et al. Society of Abdominal Radiology (SAR) and European Society of Urogenital Radiology (ESUR) joint consensus statement for MR imaging of placenta accreta spectrum disorders. Eur Radiol. 2020;30(6):3015-3024. doi: 10.1007/s00330-019-06617-7.
3. Society for Maternal-Fetal Medicine (SMFM). Diagnosis and management of vasa previa. Am J Obstet Gynecol. 2015;213(5):615-622. doi: 10.1016/j.ajog.2015.08.031.