Maternal Structures & Cervix

Cervical length (CL) measurement is most reproducible by transvaginal ultrasound. Routine Level II scans should include at least a screening assessment, with targeted transvaginal imaging in patients at increased risk.

Technique (transvaginal CL)

• Ensure an empty bladder to avoid artificial lengthening or distortion of the cervix.
• Obtain a mid-sagittal view showing:
  • Entire endocervical canal.
  • Internal and external os clearly visualized.
• Measure the closed portion of the canal from internal to external os along the endocervical line.
• Minimize probe pressure; allow the cervix to “rest” before measuring.
• Record the shortest best of at least 3 technically adequate measurements.

For Level II reporting, it is helpful to clearly document: technique (TV vs TA), gestational age, absolute CL, any funneling or dynamic changes, and whether a cerclage is present.

In patients with contractions, pressure, or suspected preterm labor, cervical imaging helps distinguish true change from stable cervix. The focus is on dynamic behavior rather than a single static number.

Key sonographic considerations

• Perform serial measurements over several minutes, observing for:
  • Shortening of the closed canal.
  • Development or progression of funneling.
  • “Sludge” or debris at the internal os.
• Use minimal probe pressure and avoid overdistension of the bladder.
• Note any bulging or prolapse of membranes into the canal or vagina.

Reporting elements

• Shortest documented CL and presence/degree of funneling (e.g., T/Y/V/U-shaped canal).
• Whether the cervix is dynamically changing during the scan.
• Any prior cerclage, pessary, or cervical surgery.

Ultrasound findings are interpreted in conjunction with symptoms, exam findings, and biochemical testing (where available). This module emphasizes imaging descriptors rather than specific treatment recommendations.

Ultrasound is used both in decisions about placing a cerclage and in evaluating the cervix afterwards. The emphasis here is on what to document rather than when to intervene.

Pre-cerclage (ultrasound information to document)

• Cervical length (TV), funneling, and dynamic change.
• Gestational age and fetal presentation.
• Membrane position, presence of “sludge,” and any prolapse beyond the internal os.
• Placental location and vasa previa screening (relevant for route and safety of placement).

Post-cerclage

• Document cerclage location and appearance; confirm that the internal os is supported.
• Measure residual cervical length above and below the suture if feasible.
• Note any funneling to or past the level of the stitch.

Many societies reference cervical length thresholds (e.g., <25 mm in women with prior spontaneous preterm birth) when considering cerclage, but the final decision incorporates history, timing, anatomy, and local expertise.

The lower uterine segment (LUS) and cesarean scar region can be evaluated transabdominally and, when needed, transvaginally. While specific thickness cutoffs remain an area of active research, descriptive assessment is useful in patients with prior cesarean or other uterine surgery.

What to assess

• Location and thickness of the LUS in the sagittal plane.
• Myometrial continuity and any focal outpouching (niche or dehiscence).
• Relationship of the scar region to the bladder and cervix.
• In late pregnancy, overlap with placental location (refer to Placenta & Cord module for previa/accreta details).

Emerging literature explores associations between thin or dehiscent LUS and uterine rupture risk, but numerical thresholds vary among studies. Many reports use a descriptive approach (e.g., “markedly thinned LUS with focal bulging”) combined with clinical risk factors when counseling patients.

When patients present with bleeding, ultrasound assessment of the cervix and lower uterine segment helps contextualize findings related to placenta, membranes, and maternal structures.

Key imaging points

• Placental edge to internal os distance:
  • Measure in the sagittal plane along the canal; avoid oblique cuts when possible.
  • Transvaginal imaging is often needed to accurately define the relationship in suspected previa.
• Cervical canal: presence of clots, membranes, or tissue.
• Lower uterine segment: evaluate for hematoma, contraction “pseudoprevias,” or focal myometrial abnormalities.

Detailed classification of placenta previa, low-lying placenta, and placenta accreta spectrum is covered in the dedicated Placenta & Cord module. This section emphasizes the imaging relationship between the cervix and maternal lower segment structures.

Congenital uterine anomalies may be first recognized during Level II ultrasound. Although 3D ultrasound and MRI provide the most definitive classification, a systematic 2D survey can suggest the anomaly type and guide counseling.

Imaging considerations

• Assess the external fundal contour (if visible) and the endometrial cavity shape.
• Note whether there is a single cervix or two cervices.
• Look for a midline fundal cleft (bicornuate) versus a largely normal fundal contour (septate).

Common patterns

• Septate uterus: normal or nearly normal outer fundal contour with an internal septum dividing the cavity.
• Bicornuate uterus: fundal indentation >1 cm with two divergent horns.
• Uterus didelphys: essentially duplicated uterine horns and cervices, sometimes with vaginal septum.
• Unicornuate uterus: asymmetric small uterus, sometimes with a rudimentary horn.

Many uterine anomalies are associated with increased risks of malpresentation, preterm birth, and growth restriction. Ultrasound in pregnancy focuses on documenting the anomaly, monitoring growth and cervical length, and planning delivery rather than definitive classification alone.

Fibroids (leiomyomas) and adnexal masses are frequently encountered during Level II scans. Most are benign but may affect pregnancy depending on size, location, and complications such as degeneration or torsion.

Uterine fibroids

• Describe number, size (3 dimensions), and location (submucosal, intramural, subserosal, cervical, lower segment).
• Note proximity to the internal os or lower uterine segment, especially for planned vaginal delivery or cesarean.
• Degeneration may present with heterogeneous echotexture and pain but is typically self-limited.

Adnexal masses

• Most simple cysts <5–6 cm are likely functional and may resolve spontaneously.
• Complex or solid components, papillary projections, or marked vascularity warrant closer follow-up and consultation.
• Ovarian torsion is suggested by acute pain and an enlarged, edematous ovary with peripheral follicles and possibly absent or reduced flow.

In pregnancy, management decisions for fibroids and adnexal masses are individualized. For Level II reports, clear documentation of size, morphology, and relationship to the uterus and cervix is most useful.