17.1 Standard Biometry: BPD, HC, AC, FL & Growth Assessment

ARRT Sonography Primary

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1 Introduction & Exam Overview

1.1 Current Exam Facts & Logistics 1.2 Scoring, Section Minimums & the August 2026 Cut-Score Change 1.3 Official Blueprint & Study Strategy

2 Patient Care: Ethics, Legal Practice & Communication

2.1 Ethical & Legal Aspects of Patient Care 2.2 Interpersonal Communication & Patient Education 2.3 Physical Assistance, Monitoring & Sonographer Ergonomics

3 Patient Care: Safety, Emergencies & Infection Control

3.1 Medical Emergencies & Critical Findings 3.2 Infection Control & Hazardous Materials 3.3 Ultrasound Safety & Interventional Procedure Care

4 Image Production: Transducers & the Ultrasound Beam

4.1 Piezoelectric Effect & Transducer Construction 4.2 Beam Configuration: Near Field, Far Field & Focus 4.3 Transducer Types, Selection, Care & Malfunctions

5 Image Production: Pulse Characteristics & Resolution

5.1 Pulse Characteristics: PRF, PRP, SPL & Duty Factor 5.2 Resolution: Axial, Lateral, Temporal, Elevational & Contrast 5.3 Frequency, Bandwidth, Q Factor, Power, Pressure, Intensity & Amplitude

6 Image Production: Imaging Modes & Doppler Principles

6.1 B-Mode & M-Mode Imaging 6.2 Spectral Doppler: Pulsed Wave vs Continuous Wave 6.3 Color Doppler & Power Doppler

7 Image Production: Optimization Controls

7.1 Power, Gain, TGC, Depth & Focal Zone 7.2 Frame Rate, Line Density, Dynamic Range, Harmonics & Compounding 7.3 Doppler Optimization Controls

8 Image Production: Sound–Tissue Interactions

8.1 Speed of Sound & the Range Equation 8.2 Acoustic Impedance, Reflection, Refraction & Scattering 8.3 Attenuation, Absorption & Intensity

9 Image Production: Signal Processing, Display & Bioeffects

9.1 Receiver, Signal Processing & Analog-Digital Conversion 9.2 Postprocessing, Magnification, Panoramic & 3D/4D Imaging 9.3 Bioeffects & Output Display Measures

10 Image Production: Image Evaluation & Artifacts

10.1 Criteria for Diagnostic Quality & Annotation 10.2 Grayscale Artifacts 10.3 Doppler Artifacts

11 Image Production: Real-Time Interpretation & Informatics

11.1 Real-Time Imaging, Echogenicity & Echotexture 11.2 Color & Spectral Analysis: Measurements and Waveforms 11.3 Imaging Informatics: HIS, RIS, EMR, PACS & DICOM

12 Procedures: Abdominal & Transplant Vasculature

12.1 Aorta, Its Branches & the IVC 12.2 Portal Venous System & Hepatic Vasculature 12.3 Liver & Renal Transplant Sonography

13 Procedures: Liver

13.1 Liver Anatomy, Variants & Examination Technique 13.2 Diffuse Liver Disease 13.3 Focal Liver Lesions

14 Procedures: Biliary System, Pancreas & Spleen

14.1 Gallbladder & Bile Ducts 14.2 Pancreas 14.3 Spleen

15 Procedures: Urinary Tract, GI Tract & Other Abdominal Structures

15.1 Kidneys & Ureters 15.2 Bladder & Prostate 15.3 GI Tract, Adrenals, Lymph Nodes, Hernias & Peritoneal Cavity

16 Procedures: First-Trimester Obstetrics

16.1 Early Gestation: Normal Development & Structures 16.2 First-Trimester Measurements, Dating & Maternal Anatomy 16.3 First-Trimester Abnormalities

17 Procedures: Second/Third-Trimester Biometry & Fetal Survey

17.1 Standard Biometry: BPD, HC, AC, FL & Growth Assessment 17.2 Fetal Anatomic Survey: Head, CNS, Face & Spine 17.3 Fetal Anatomic Survey: Heart, Chest, GI, GU, Skeleton & Fetal Position

18 Procedures: Chromosomal, Genetic & Structural Abnormalities

18.1 Chromosomal & Genetic Abnormalities 18.2 Major Structural Anomalies by System 18.3 Fetal Infection, Neoplasms & Hydrops

19 Procedures: Placenta, Amniotic Fluid, Cord & Cervix

19.1 Placenta: Normal, Previa, Abruption & Accreta 19.2 Amniotic Fluid Assessment & Growth Disorders 19.3 Umbilical Cord & Cervical Assessment

20 Procedures: Fetal Assessment, Doppler & Multiple Gestation

20.1 Biophysical Profile & Fetal Surveillance 20.2 Fetal & Uteroplacental Doppler 20.3 Multiple Gestations, Assisted Reproduction & Maternal Disease

21 Procedures: Gynecology — Uterus

21.1 Uterine Anatomy, Myometrium & Fibroids 21.2 Endometrium: Cyclic Changes, Hyperplasia & Cancer 21.3 Cervix & Congenital Uterine Anomalies

22 Procedures: Gynecology — Adnexa & Pelvis

22.1 Ovaries: Normal, Functional Cysts & Benign Masses 22.2 Ovarian Malignancy, Torsion & Fallopian Tubes 22.3 Cul-de-Sac, Vagina & Pelvic Floor

23 Procedures: Neck & Scrotum

23.1 Thyroid & Parathyroid 23.2 Salivary Glands & Neck Lymph Nodes 23.3 Scrotum & Testes

24 Procedures: Breast & Peripheral Vascular

24.1 Breast & Axilla 24.2 Venous Extremity Doppler 24.3 Carotid Doppler & Post-Catheterization Complications

25 Procedures: Pediatric, Interventional & Miscellaneous

25.1 Neonatal Head, Spine & Hip Sonography 25.2 Pediatric GI & Noncardiac Chest 25.3 Ultrasound-Guided Interventions, MSK & Superficial Masses

17.1 Standard Biometry: BPD, HC, AC, FL & Growth Assessment

Key Takeaways Explain This Topic AI

The BPD and HC are measured in the same axial plane at the level of the thalami and cavum septi pellucidi, with the third ventricle midline

The abdominal circumference is taken at the level of the fetal stomach and the J-shaped umbilical vein/portal sinus, with the spine at 3 or 9 o'clock

Femur length measures the ossified diaphysis only; the distal femoral epiphysis is never included

Hadlock estimated fetal weight equations combine BPD/HC, AC, and FL and carry an inherent error of roughly ±15–18%

• Fetal growth restriction is screened by an AC or EFW below the 10th percentile for gestational age
• Last updated: July 2026

Standard fetal biometry is the quantitative backbone of every second- and third-trimester sonogram. Four core measurements — the biparietal diameter (BPD) , head circumference (HC) , abdominal circumference (AC) , and femur length (FL) — are combined to estimate gestational age, calculate estimated fetal weight (EFW) , and screen for abnormal growth. The ARRT exam expects you to know the exact anatomic landmarks for each plane, because a measurement taken in the wrong plane is the single most common source of biometric error.

Head Biometry: BPD and HC

The BPD and HC are obtained in the same axial (transaxial) plane through the fetal head. The correct plane is identified by these landmarks:

• A smooth, symmetric, oval calvarium with a continuous hyperechoic rim
• The cavum septi pellucidi (CSP) anteriorly, appearing as a small fluid-filled box between the frontal horns
• The paired thalami in the midbrain, with the slit-like third ventricle between them on the midline

No cerebellum in the image — if the cerebellum is visible, the plane is too low (that is the transcerebellar plane used for the cisterna magna, not for BPD)

The BPD is measured at the widest portion of the skull, perpendicular to the midline falx, from the outer edge of the near (proximal) calvarium to the inner edge of the far (distal) calvarium — the classic outer-to-inner technique. The HC is traced (or calculated from the BPD and occipitofrontal diameter) around the outer table of the calvarium at the same level, excluding the scalp. Of the head measurements, the HC is the most reproducible and is least affected by head-shape variation.

Cephalic Index

Because head shape varies, the cephalic index (CI) checks whether the BPD alone is a trustworthy age predictor:

• Parameter Formula / Value Interpretation
• Cephalic index (BPD ÷ OFD) × 100 Normal ≈ 75–85
• Dolichocephaly CI < 70–75 Elongated head (breech, oligohydramnios); BPD underestimates age
• Brachycephaly CI > 85 Rounded head; BPD overestimates age; soft marker for trisomy 21

When the CI is abnormal, the sonographer should rely on the HC rather than the BPD for age and growth equations.

Abdominal Circumference

The AC is the single most important measurement for detecting growth disorders because it reflects fetal liver size and subcutaneous fat. The correct transverse (axial) plane of the abdomen shows:

The stomach bubble in the left upper quadrant

The umbilical vein/portal sinus coursing in a short J-shape within the liver (a long vein segment means the plane is too oblique)

• The spine at the 3 o'clock or 9 o'clock position , which guarantees a true axial cut
• A round (not elliptical) abdominal outline

The plane must be above the kidneys and the cord insertion — both of which falsely enlarge the circumference — and below the heart. The AC is traced around the outer skin line or calculated from perpendicular AP and transverse diameters.

Femur Length

The FL is measured along the long axis of the femoral shaft with the beam perpendicular to the bone. Only the ossified diaphysis is included, from one blunt end to the other; the cartilaginous epiphyses and the distal femoral epiphysis (a secondary ossification center appearing after ~32 weeks) are excluded. A bright distal reflection at the far end (the "distal point") should not be included either. FL is the most resistant of the four parameters to head- or body-shape distortion, but it shortens in skeletal dysplasias and may be mildly short as a soft marker for trisomy 21 .

Estimated Fetal Weight and Dating

Hadlock formulas estimate fetal weight by combining AC with head and femur data using logarithmic equations — for example, log₁₀(EFW) from BPD, AC, and FL, or from HC, AC, and FL. Because errors compound across parameters, EFW carries an inherent accuracy of only about ±15% (roughly two standard deviations) in the third trimester, and it should be reported with a percentile, not as a precise number.

Biometric dating accuracy declines as pregnancy advances because individual growth variation widens:

• Second trimester (14–27 weeks): approximately ±1–2 weeks
• Third trimester (≥28 weeks): approximately ±2–3 weeks

A composite age (averaging all parameters) improves precision, but pregnancies should be dated by the earliest reliable scan; a third-trimester examination should never be used to redate a well-dated pregnancy.

Growth Assessment and Ratios

Fetal growth restriction (FGR) is screened sonographically when the AC or the EFW falls below the 10th percentile for gestational age; macrosomia is suspected above the 90th percentile (EFW > 4,500 g at term). Symmetric (type I) FGR reduces all parameters proportionally and suggests an early insult (aneuploidy, infection), while asymmetric (type II) FGR — the more common uteroplacental pattern — spares the head (brain-sparing) while the AC and weight lag. Two ratios support this assessment: the HC/AC ratio , which normally falls from about 1.18 early in the second trimester toward 1.0 near term (an elevated HC/AC signals asymmetric FGR), and the FL/AC ratio , which stays nearly constant at about 22% (normal 20–24%) after 22 weeks and is gestational-age independent. Document cardiac activity, fetal number, and presentation alongside biometry on every study.