Ch 20 OB/GYN

Chapter 20: Assessment of Fetal Age and Size in the Second and Third Trimester

## Author: Susan Raatz Stephenson

Objectives
  • Recognize the correct level for obtaining standard second- and third-trimester biometric measurements.
  • Explain the use of multiple biometric parameter accuracy in fetal dating.
  • Summarize the use of alternate dating methods such as humerus, forearm, lower leg, cerebellar, and ocular measurements.
  • Discuss the correct method for measuring fetal biometry.
  • Calculate estimated fetal weight.

- List fetal ratios that help determine normal growth of the fetus.

Standard Routine Measurements
  1. Biparietal Diameter (BPD)
  2. Head Circumference (HC)
  3. Abdominal Circumference (AC)

4. Femur Length (FL)

Head Measurements
  1. Biparietal Diameter (BPD)
  2. Head Circumference (HC)
  3. Cephalic Index (CI)
  4. Transverse Cerebellum

5. Binocular Measurement

Biparietal Diameter (BPD)
  • Accuracy of landmarks is critical for measuring.
  • Optimal image should be taken directly over fetal head in transverse position.
  • BPD Measurement Details:

- Measure perpendicular to the fetal skull at the level of thalamus and cavum septum pellucidum.

BPD Landmarks
  • Cranial Sutures
  • Frontal Bone
  • Parietal Bone
  • Falx Cerebri
  • Cavum Septum Pellucidum (CSP)
  • Thalamus
  • Occipital Bone

- Third Ventricle

BPD Measurement Technique
  • The head should be ovoid, avoiding brachycephaly (round) or dolichocephaly (elongated).
  • Calipers placed from the leading edge of one parietal bone to the opposite parietal bone; measurement taken from outer edge to inner edge.

- Include only skull bone in the measurement; soft tissue should not be included.

Head Circumference (HC)
  • Fetal Skull Compression
    • Conditions that lead to compression include:
    • Normal vertex position
    • Uterine tumors
    • Intrauterine crowding

- HC is less affected by head compression than BPD.

HC Measurement Process
  • Measurement should occur on the same image as BPD.
  • Measure HC from the outer margin of the nearest margin of the calvaria to the inner margin of the farthest calvaria margin.

- The references for measurement are thalamus and CSP.

Cephalic Index (CI)
  • Defines head shape.
  • CI Categories:
    • Dolichocephalic: CI < 75%
    • Mesocephalic: CI 75-80%
    • Brachycephalic: CI > 80%
  • Normal Range for CI: 76-90%
    • Brachycephaly: CI > 90%

- Dolichocephaly: CI < 76%

Cerebellum Measurements

- Measure the transverse cerebellar diameter considering the cerebellar hemispheres (1), cisterna magna (2), and falx anatomy.

Binocular Measurement
  • Focus on fetal orbits:

- Includes nasal bone (1), lenses of the eyes (2), and binocular distance (x).

Abdominal Measurements
  • Anatomical Landmarks:
    • Junction of umbilical vein/portal vein
    • Stomach
    • Aorta
    • Spine

- Should present a circular-shaped abdomen.

Abdominal Circumference (AC)
  • Major uses include monitoring normal fetal growth and detecting growth retardation or macrosomia.
  • AC is used to determine growth/size, not age.
  • Measurement is taken in the transverse plane at the level of liver where the umbilical vein branches into left portal.

- Growth patterns cause significant variability in AC measurements.

AC Landmarks
  • J-shape formed by the LPV and umbilical vein.
  • Stomach positioned on the left side.
  • Abdomen should ideally be circular in shape; oval shape may lead to an overestimate of size.
  • Visual presence of kidneys may indicate measurement is too low.
  • Changes in shape can result from breathing, compression, crowding, and position changes.

- Multiple measurements are recommended for accuracy.

Long Bones
  1. **Femur
  2. Humerus
  3. Tibia/Fibula

4. Radius/Ulna**

Femur Length (FL)
  • Femur is always measured due to its high accuracy in determining gestational age.
  • The transducer must be rotated until the entire length of the femur is visible.

- Ends of the femur should be seen as blunt, not pointed.

FL Measurement Techniques

- Measurement includes only the ossified portions from the proximal end near the femoral neck to the distal epiphyses.

Extremities Detailed Measurement
  • Humerus
    • More difficult to measure than the femur.
    • Anterior humerus might be too superficial to image; posterior humerus may be obscured by rib and spine shadows.
  • Radius and Ulna
    • Two parallel bones observed distal to the humerus; rotations aid in elongation during measurements.
    • Commonly assessed by observing the fetal face.

- Ulna is larger and located more medially than the radius.

Detailed Humerus Measurement Technique
  • Care should be exercised as the humerus may appear similar to the femur.

- Measurements must be taken distinctly from adjacent bony thorax through the anechoic humeral head (H) into the arm; measurement should be noted with calipers.

Tibia/Fibula Measurement Process
  • Illustrated by a long-axis view of the lower leg demonstrating ossified portions of the fibula and tibia.

- Fibula is visually located lateral to the tibia.

Radius/Ulna Measurement Process
  • Forearm measurements show the ulna extending further into the elbow than the radius, with both bones terminating at approximately the same wrist level.
  • The absence of wrist bone ossification leads to a hypo to anechoic area between forearm and hand.