Placenta & Umbilical Cord

Overview

The placenta and umbilical cord provide the critical interface for maternal–fetal exchange. Level II ultrasound evaluation includes placental location, morphology, cord insertion, and cord structure and vessels.

This module focuses on structural and sonographic findings. Clinical management of hemorrhage and placenta accreta spectrum (PAS) is addressed in dedicated condition and protocol pages.

Normal placental thickness at mid-gestation is approximately 2–4 cm at the cord insertion, with a homogeneous texture and smooth chorionic plate. Mild placental lakes are common and usually benign.

Placenta previa

Placenta previa refers to a placenta that partially or completely overlies the internal cervical os. Accurate classification and measurement are essential for counseling about the risk of antepartum hemorrhage and mode of delivery.

Definitions (sonographic)

• Complete previa: Placental tissue completely covers the internal os.
• Partial / marginal previa: Placental edge reaches or partially covers the internal os.
• Low-lying placenta: Placental edge is near, but not covering, the internal os (see below).

Technique

• Transvaginal ultrasound is preferred when placenta previa is suspected; it is safe and provides more accurate measurement.
• Measure the distance from the placental edge to the center of the internal os along the cervical canal.
• A partially filled bladder helps avoid overestimation or underestimation of the distance.

Clinical importance

• Risk of antepartum bleeding, preterm birth, and cesarean delivery.
• Increased risk of placenta accreta spectrum when placenta previa coexists with prior uterine surgery (e.g., Cesarean scar).
• Most low-lying placentas diagnosed in the second trimester will migrate away from the os as the lower uterine segment develops.

Low-lying placenta

A low-lying placenta is typically defined when the placental edge is close to, but not covering, the internal os. Definitions vary by guideline; many use a cutoff of approximately <20 mm from the os in the third trimester. Local protocols should be followed.

Key points

• Common finding in the second trimester; most will move away from the os as the uterus grows.
• Repeat imaging in the third trimester is recommended when the placenta is low-lying on mid-gestation scan.
• Delivery planning should consider placental edge distance, bleeding history, and obstetric history.

The terms and thresholds for “low-lying” versus “previa” may differ among guidelines. This page summarizes structural concepts rather than prescribing specific management cutoffs.

Placenta accreta spectrum (summary)

Placenta accreta spectrum (PAS) includes placenta accreta, increta, and percreta, characterized by abnormal adherence or invasion of placental villi into the myometrium and beyond. It is strongly associated with placenta previa and prior uterine surgery (particularly previous Cesarean delivery).

Key ultrasound features (summary)

• Loss or thinning of the hypoechoic retroplacental zone.
• Numerous placental lacunae giving a “moth-eaten” or “Swiss cheese” appearance.
• Irregular uterine–bladder interface.
• Abnormal placental vascularity on color Doppler, including bridging vessels.

Detailed diagnostic criteria, MRI findings, and management (timing and mode of delivery, multidisciplinary planning) are covered in the dedicated module:
Placenta accreta spectrum — clinical management (proposed URL).

Succenturiate and bilobed placenta

A succenturiate placenta has one or more accessory lobes separate from the main placental disk but connected by membranous vessels. A bilobed placenta has two nearly equal lobes.

Sonographic features

• Separate placental lobes connected by thin membranes.
• Vessels traversing membranes between lobes; color Doppler can help delineate.
• Cord insertion may be into the main lobe or an accessory lobe.

Clinical considerations

• Increased risk of vasa previa when vessels traverse the internal os.
• Risk of retained placenta and postpartum hemorrhage from retained accessory lobe.
• Careful assessment of cord insertion and vessels near the internal os is essential.

Circumvallate and circummarginate placenta

A circumvallate placenta has an elevated, rolled edge of the chorionic plate, with membranes inserting inward from the placental margin. Circummarginate placenta is a less pronounced variant.

Sonographic appearance

• Irregular, rolled chorionic plate edge.
• Peripheral placental shelf or ridge projecting into the amniotic cavity.
• Membranes inserting inward from the edge rather than at the placental margin.

Older literature associated circumvallate placenta with growth restriction, bleeding, and preterm birth; more recent data suggest many cases may have a benign course. Local practice patterns vary.

Placental cysts and chorioangioma

Placental cysts

Small anechoic or hypoechoic placental cysts are relatively common and usually benign, especially when isolated and away from the cord insertion.

• Subchorionic or intraplacental, usually <2–3 cm.
• May represent intervillous or subchorionic fluid collections.
• Isolated small cysts typically have no adverse effect.

Chorioangioma

A placental chorioangioma is a benign vascular tumor of the placenta. Larger lesions can be associated with significant fetal and maternal complications.

• Well-circumscribed, heterogeneous mass, usually near the cord insertion.
• Increased vascularity on color Doppler.
• Large tumors may be associated with polyhydramnios, fetal anemia, hydrops, and growth restriction.

Management of large chorioangiomas may involve close surveillance and, in selected cases, fetal therapy (e.g., laser ablation or embolization) at specialized centers.

Subchorionic hematoma / subchorionic hemorrhage

A subchorionic hematoma is a collection of blood between the chorionic membrane and the uterine wall. It may be incidental or associated with first- or second-trimester bleeding.

Sonographic features

• Hypoechoic or heterogeneous collection adjacent to the gestational sac or placental margin.
• Appearance may evolve over time as blood products age.
• Large collections may partially separate the placenta from the uterine wall.

Outcome depends on size, gestational age, and associated symptoms. A dedicated clinical module can provide more detail on prognosis and management.

Marginal cord insertion

In a marginal insertion (sometimes called battledore placenta), the umbilical cord inserts within a short distance of the placental edge rather than centrally.

Sonographic features

• Cord insertion within approximately 2 cm of the placental edge (cutoff may vary by guideline).
• Vessels entering the placenta tangentially rather than centrally.

Clinical considerations

• Usually benign when isolated.
• May be associated with growth restriction or abnormal Doppler in some series.
• Careful evaluation of lower uterine segment is warranted if the insertion is near the internal os.

Velamentous cord insertion

In a velamentous insertion, the umbilical vessels leave the placental disk and traverse the membranes unsupported by Wharton jelly before reaching the placental edge or the uterine wall.

Sonographic features

• Cord insertion into the membranes rather than directly into the placental disk.
• Unprotected vessels coursing between placenta and internal os or along the membranes.
• Color Doppler confirms vascular flow in these membranous vessels.

Clinical significance

• Associated with vasa previa when vessels cross the internal os.
• Increased risk of fetal growth restriction and abnormal fetal heart rate tracings in labor.
• Delivery planning is guided by presence or absence of vasa previa and other obstetric factors.

A more detailed discussion of velamentous insertion and its relationship to vasa previa will be provided in related sub-modules (e.g., insertion anomalies, vasa previa).

Vasa previa (summary)

Vasa previa is a condition in which fetal vessels traverse the membranes in the lower uterine segment, passing over or near the internal cervical os, unprotected by placental tissue or Wharton jelly. Rupture or compression of these vessels can cause rapid fetal exsanguination.

Types

• Type I: Velamentous cord insertion with vessels crossing the internal os.
• Type II: Vessels connecting a succenturiate or bilobed placenta crossing the os.
• Type III: Vessels leave and returning to the placental edge following a path described by the shape of a boomerang.
  

Sonographic evaluation

• Targeted transvaginal scan when suspected from transabdominal imaging or risk factors (low-lying placenta, succenturiate lobe, IVF pregnancy).
• Use color Doppler to identify vessels crossing or near the internal os.
• Confirm fetal origin of the vessels with pulsed Doppler waveform (fetal heart rate, not maternal).

Clinical considerations (summary)

• Associated with significant risk of fetal mortality if undiagnosed before labor or membrane rupture.
• Once confirmed, planned Cesarean delivery before labor and membrane rupture is typically recommended.
• Detailed clinical guidance is covered in the dedicated vasa previa module.

Full discussion of screening strategies, timing of delivery, and management will be provided in:
Vasa previa — detailed ultrasound and management (proposed URL).

Single umbilical artery (SUA) — summary

A single umbilical artery (SUA) is the most common umbilical cord abnormality, characterized by one umbilical vein and only one umbilical artery.

Diagnosis

• Cross-section of the cord shows one vein and one artery instead of two arteries.
• Color Doppler in the fetal pelvis (around the bladder) demonstrates a single umbilical artery adjacent to the bladder instead of the usual paired arteries.

Clinical significance

• May be isolated, in which case prognosis is often good.
• Can be associated with structural anomalies (cardiac, renal, musculoskeletal) and chromosomal abnormalities.
• Detailed anatomic survey and growth surveillance are recommended.

A full review of epidemiology, associated anomalies, and recommended follow-up will be presented in the dedicated SUA module:

Cord knots and nuchal cord

True and false knots

True knots represent actual looping and tightening of the cord on itself. False knots are focal redundancies or kinking of vessels without a true knot.

• True knot may appear as a cloverleaf or clump of cord on 2D imaging; 3D and color Doppler can assist.
• False knots are usually benign and of no clinical consequence.

Nuchal cord

A nuchal cord is one or more loops of cord around the fetal neck. It is common and usually an incidental finding.

• Color Doppler and sagittal/axial views of the neck can demonstrate cord loops.
• Most nuchal cords are not associated with adverse outcome.
• Management decisions are guided by overall fetal status and intrapartum monitoring, not ultrasound alone.

Cord length, coiling, and torsion

Umbilical cords vary in length and coiling. Very short cords or excessively long cords have been associated with adverse pregnancy outcomes in some studies (abruption, fetal distress, entanglement), but most cords fall within a broad normal range.

Sonographic considerations

• Global measurement of cord length is usually not feasible on routine ultrasound.
• Focal abnormalities such as segmental torsion or stricture can sometimes be identified.
• Excessive or absent coiling may be seen but is not routinely quantified in most practices.

At present, sonographic assessment of cord length and coiling is not standardized. Apparent abnormalities should prompt careful evaluation of fetal growth, Doppler, and well-being rather than being interpreted in isolation.

References

Selected classic and commonly cited references for placental and cord abnormalities are listed in abbreviated form below. Additional contemporary guideline documents should be consulted for management recommendations.

1. Gilbert WM, Nicolaides KH. Fetal omphalocele and associated malformations and chromosomal defects. Obstet Gynecol. 1987;70:633–635.
2. Crawford RA, Ryan G, Wright VM, et al. Importance of serial biophysical assessment of fetal wellbeing in gastroschisis. Br J Obstet Gynaecol. 1992;99:899–903.
3. Yetter JF. Examination of the placenta. Am Fam Physician. 1998;57(5):1045–1054.
4. Resnik R. Management of placenta accreta, increta, and percreta. Classic reviews.
5. Jauniaux E, et al. Placenta accreta spectrum disorders: prenatal diagnosis and management. Review articles and guidelines.
6. Catanzarite V, et al. Prenatal diagnosis of vasa previa: sonographic and clinical findings.
7. Sepulveda W, et al. Velamentous insertion of the umbilical cord and vasa previa: diagnosis and outcome.
8. Bromley B, Benacerraf BR. Placental lakes and placental sonolucencies. J Ultrasound Med. Classic review.
9. Benirschke K, Kaufmann P. Pathology of the Human Placenta. Textbook reference.