What structures should be seen at the level at which the Biparietal diameter is measured?

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The important structures in the fetal head can be obtained using an axial plane.  After obtaining the first set of images from the BPD level, rotation of the transducer will bring the cerebellum and the posterior fossa in to view.

BPD Level Structures
Cerebellum Level Structures

Axial Plane – BPD level

BPD Level Structures | © 2013 Australasian Society for Ultrasound in Medicine

1. Biparietal diameter (BPD)
2. Cavum Septum Pellucidum (CSP)
3. Thalami
4. Hypoechoic skull sutures
5. Third ventricle
6. Choroid
7. Posterior lateral ventricles
8. Corpus callosum

KEY POINTS:

BPD

• For optimal BPD, obtain an axial image, ensuring
  • 1 caliper is placed outside of the skull bone, and contralateral caliper is placed on the inside of the skull bone
  • Always be sure the midline falx is present
    • Should be contiguous from front to back of the skull (except for the cavum septum pellucidum [CPS])
    • Should be equidistant from both front and back of the skull
    • Abnormal falx is a concern and may indicate holoprosencephaly

CSP

PLEASE INSERT FIGURE 3: Caption Cavum Septum Pellucidum (CSP) and Corpus Collosum

• Never clear a fetal head ultrasound without seeing a CSP
  • CSP box should always be clear without a bisecting line down the middle
  • An abnormal CSP is a marker for abnormal forebrain development
  • Also note the corpus collosum, which should appear as a hypoechoic region anterior to the CSP

Ventricles

• You can measure lateral ventricles either
  • Mid-choroid (common radiologist method) or
  • Tip of the choroid (common MFM method)
• Normal: <10 mm
  • See ‘ObG Related Topics’ below for clinical guidelines
• Choroid plexus ‘cyst’ (CPC)
  • Not a true cyst but collection of fluid
  • Usually benign and resolves
  • Ensure normal fetal growth, open hands and absence of other anomalies that may suggest more serious syndromes (e.g., Trisomy 18)
  • See ‘ObG Related Topics’ below for clinical guidelines

Axial Plane – Cerebellum level

©Cerebellum Level Structures | 2013 Australasian Society for Ultrasound in Medicine

1. Cavum septum pellucidum (CSP)
2. Cerebellar lobes
3. Cerebellar vermis
4. Cisterna magna
5. Nuchal fold
6. Cerebellar peduncles
7. Falx
8. Thalami

KEY POINTS:

Posterior Fossa

Cerebellum

• Transverse cerebellar measurement (in millimeters) correlates with gestational age until 20 weeks
  • Can be a good adjunct for confirming pregnancy dates
  • Never ignore a cerebellum measuring >2 weeks behind
• A crowded posterior fossa or “banana” shaped cerebellum should raise suspicion for spina bifida

Cisterna Magna

• Cisterna magna should be measures from vermis to inside of skull bone
  • Normal: 2-10 mm

Nuchal Fold

• Nuchal fold may be measured up 22 weeks
  • Normal: >6 mm
  • If >6 mm, consider Trisomy 21 testing
  • See ‘Related ObG Topics’ for clinical guidelines

Important to Note

• If ever in doubt as to whether findings are ‘within normal’, refer for specialized (also known as targeted or detailed) ultrasound
• ACOG/SMFM recommends offering invasive testing using microarray for fetal structural anomalies detected on prenatal ultrasound

Learn More – Primary Sources:

Obstetric and gynecologic ultrasound curriculum and competency assessment in residency training programs: consensus report 

AIUM Practice Parameter for the Performance of Obstetric Ultrasound Examinations

ISUOG: Performance of the routine mid-trimester fetal ultrasound scan

Locate a Maternal Fetal Medicine Specialist:

Maternal-Fetal Medicine Specialist Locator-SMFM

Locate a genetic counselor or genetics services:

Genetic Services Locator-ACMG

Genetic Services Locator-NSGC

Genetic Services Locator-CAGC

Related ObG Topics:

What does the Biparietal diameter measure?

At what level is the Biparietal diameter most accurately measured?

What are the landmarks for the measurement of the head circumference and BPD?

How is BPD measured?