Placenta Ultrasound: Location, Previa, and What to Document

Placental location is a standard component of every second and third trimester ultrasound. A low-lying placenta or placenta previa has significant clinical consequences for delivery planning, so your documentation needs to be accurate and complete. Here's how to approach it.

Normal Placenta

Appearance:

• Homogeneously echogenic structure (isoechoic to slightly hyperechoic compared to myometrium)
• Smooth maternal surface (basal plate at the myometrium)
• Smooth fetal surface (chorionic plate, with umbilical cord insertion)
• Thickness: typically 2–4 cm (1 mm per week of gestation — rough rule)

What to document:

• Location (anterior / posterior / fundal / right lateral / left lateral / low-lying)
• Relationship of the lower placental edge to the internal cervical os
• Grade (Grannum grading — optional, less commonly required now)
• Umbilical cord insertion site (central / eccentric / marginal / velamentous)

Placental Location Terminology

Term	Definition
Anterior	Placenta on the front uterine wall
Posterior	Placenta on the back uterine wall
Fundal	Placenta at the top of the uterus
Low-lying	Lower placental edge within 2 cm of internal os but not covering it
Marginal previa	Lower placental edge reaches but does not cover the internal os
Partial previa	Placenta partially covers the internal os
Complete previa	Placenta completely covers the internal os

Note: The terms "marginal," "partial," and "complete" previa are being replaced in some guidelines with simply "placenta previa" (when the placenta covers the os) vs "low-lying placenta" (within 2 cm but not covering). Check your department's reporting preference.

Why Placental Location Matters at Different Gestational Ages

18–20 weeks (anatomy scan): A low-lying placenta at 20 weeks is very common and usually resolves by term as the lower uterine segment grows — a process called "placental migration." Do not alarm the patient based on a 20-week finding alone.

32–36 weeks: If low-lying at 20 weeks, follow-up imaging is indicated. By 32–36 weeks, the lower uterine segment has elongated and most low-lying placentas have moved.

Term (36+ weeks): Distance to internal os is critical for delivery planning. Placenta previa at this stage requires cesarean delivery.

Measuring the Placenta-to-Os Distance

Transabdominal approach (TAS):

• Sagittal plane through the uterine cervix and lower uterine segment
• Identify the internal cervical os
• Measure from the lower placental edge to the os in mm
• TAS overestimates the distance and is less accurate than TVS

Transvaginal approach (TVS) — preferred for low-lying placenta:

• TVS is safe in placenta previa and provides far superior visualization of the os
• Insert probe gently to the vaginal fornix — no need to touch or enter the cervix
• Sagittal plane: identify the internal os as the point where the cervical canal opens into the uterine cavity
• Measure from the lower placental edge to the internal os
• If the placenta overlaps: measure how far it covers the os (report as overlap in mm)

Reporting the distance:

• 20 mm from os at 32+ weeks: vaginal delivery likely safe

• 1–20 mm (low-lying): close surveillance, likely planned cesarean
• 0 mm / overlapping: placenta previa — cesarean delivery

Anterior Placenta: Special Considerations

An anterior placenta is normal — but it has practical implications:

• Fundal height: Uterus may measure larger
• Amniocentesis: Higher risk of placental needle traversal — operator will try to find a window
• C-section: Anterior placenta with prior uterine surgery = higher risk of placenta accreta spectrum

Documenting an anterior placenta: Always note whether it is low-lying or reaching toward the lower uterine segment. An anterior placenta near the scar of a prior cesarean section warrants additional scrutiny (see accreta below).

Placenta Accreta Spectrum

Placenta accreta spectrum (PAS) refers to abnormal placental attachment to the uterine wall, ranging from accreta (superficial) to increta (into the myometrium) to percreta (through the serosa).

Risk factors:

• Prior cesarean section(s) — the most important risk factor
• Anterior low-lying placenta over the cesarean scar
• Prior uterine surgery

Ultrasound features of PAS:

• Loss of retroplacental clear zone: The normal hypoechoic strip of myometrium behind the placenta disappears
• Placental lacunae: Irregular anechoic vascular spaces within the placenta — "moth-eaten" or "Swiss cheese" appearance — highly associated with accreta
• Bridging vessels: Vessels crossing from the placenta into the myometrium on color Doppler
• Bladder wall irregularity: In percreta, vessels may extend into or through the bladder wall

Reporting language for suspected PAS: "Findings are concerning for possible placenta accreta spectrum. Loss of the retroplacental clear zone and placental lacunae are identified at the [location] where the placenta overlies the prior cesarean scar. MRI and maternal-fetal medicine consultation are recommended."

Vasa Previa

Vasa previa occurs when fetal vessels from a velamentous cord insertion or succenturiate lobe cross the internal os in front of the presenting part. This is a life-threatening fetal emergency if the membranes rupture.

Ultrasound features:

• Velamentous cord insertion (cord inserts into the membranes, not the placenta directly)
• Vessels visible overlying the internal os on color Doppler
• May see pulsatile vessels at the os on transvaginal scan

Any velamentous insertion or low marginal insertion should prompt transvaginal color Doppler evaluation of the internal os.

Key Images to Capture

1. Placenta — panoramic view showing its full extent and location
2. Lower uterine segment — sagittal, showing internal os and lower placental edge
3. Measurement of placenta-to-os distance (TVS if low-lying)
4. Cord insertion site — color Doppler
5. Retroplacental zone (for accreta risk cases)
6. Placental lacunae (if present) — with and without color Doppler

Reporting Examples

• "Posterior placenta, fundal, not low-lying. Umbilical cord inserts centrally. No placental abnormality identified."
• "Anterior placenta, extending into the lower uterine segment. Lower placental edge is 8 mm from the internal cervical os on transvaginal imaging. Findings consistent with low-lying placenta. Follow-up imaging recommended at 32–36 weeks."
• "Anterior complete placenta previa — placenta covers the internal os with 22 mm of overlap on transvaginal imaging. Prior cesarean scar is underlying the placenta. Retroplacental clear zone is absent at the scar site. Placental lacunae are identified. Findings raise concern for placenta accreta spectrum."

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SonoBuddy's OB protocols and Pathologies section covers placenta previa and accreta — including red flags and reporting tips.