Obstetric Ultrasound (Anatomy Scan)

The second-trimester anatomy (mid-trimester) ultrasound, typically performed at 18-22 weeks, is a systematic survey of fetal anatomy to detect structural anomalies and assess growth, placenta, and amniotic fluid. Standardized biometry (BPD, HC, AC, FL) estimates fetal weight and confirms dating when earlier scans are unavailable. It documents required anatomic views per practice guidelines and screens for markers of aneuploidy. It is a cornerstone of prenatal care and informs delivery planning and counseling.

Indications

Routine fetal anatomic survey at 18-22 weeks gestation
Assessment of fetal growth and estimated fetal weight
Detection and characterization of structural anomalies
Evaluation of placental location, amniotic fluid volume, and cervical length when indicated
Follow-up of findings from earlier scans or abnormal serum screening
Assessment in multiple gestations for growth and anomaly
Evaluation of maternal conditions affecting the fetus (diabetes, hypertension)

Contraindications & Cautions

No absolute contraindications; ultrasound is non-ionizing
Observe ALARA principles and keep thermal/mechanical indices low
Maternal body habitus, fetal position, and oligohydramnios may limit completeness (document and rescan)
Doppler exposure kept to the minimum necessary
Patient discomfort or inability to remain still may curtail the examination

Patient Preparation

Explain the scope and limitations of the anatomy scan and obtain consent
No fasting required; a moderately full bladder can help assess the cervix and lower uterine segment
Review dating, prior imaging, serum screening, and maternal history
Position semi-recumbent with left lateral tilt to avoid aortocaval compression
Advise that some structures may not be seen and a repeat scan can be needed
Discuss whether the parents wish to know fetal sex per local policy

Technique & Parameters

Curvilinear transabdominal probe (typically 3-5 MHz); transvaginal for cervix if indicated
Biometry: biparietal diameter and head circumference at the thalami, abdominal circumference at the stomach/portal sinus, femur length
Cardiac views: four-chamber, left and right outflow tracts, three-vessel and trachea view
Systematic survey of head, face, spine, thorax, abdomen, kidneys, bladder, and limbs
Assess placental location and relationship to the internal os and umbilical cord insertion/vessel number
Measure amniotic fluid (single deepest pocket or AFI) and document fetal number/position
Store standardized images and measurements per guideline requirements

Systematic Review

Head: skull shape, midline falx, cavum septi pellucidi, ventricles, cerebellum, cisterna magna, nuchal fold
Face: orbits, upper lip/palate, profile
Spine: longitudinal and transverse integrity of skin and vertebral elements
Chest/heart: lungs, four-chamber and outflow tract views, cardiac axis/position
Abdomen: stomach, bowel, abdominal wall/cord insertion, diaphragm
Genitourinary: both kidneys, renal pelves, bladder, and amniotic fluid
Limbs: presence, length, and morphology of all long bones and hands/feet
Placenta, cord (three vessels), cervix, and amniotic fluid volume

Key Findings & Significance

Ventriculomegaly (atrium >10 mm) may indicate CNS anomaly or aneuploidy
Absent cavum septi pellucidi suggests midline brain malformation
Lemon and banana signs indicate an open neural tube defect/Chiari II
Cardiac axis deviation or abnormal outflow tracts suggest congenital heart disease
Bilateral renal pelvis dilatation may indicate obstruction or reflux
Short femur/long bones and soft markers may raise aneuploidy risk
Low-lying placenta covering the internal os indicates placenta previa requiring follow-up

Differential Considerations

Ventriculomegaly: aqueductal stenosis, agenesis of corpus callosum, hemorrhage, infection, aneuploidy
Cystic posterior fossa: mega cisterna magna, Dandy-Walker malformation, arachnoid cyst
Echogenic bowel: normal variant, cystic fibrosis, infection, aneuploidy, swallowed blood
Renal pelvic dilatation: transient, ureteropelvic junction obstruction, vesicoureteral reflux, posterior urethral valves
Abdominal wall defect: gastroschisis (free-floating bowel) vs omphalocele (membrane-covered)
Short long bones: constitutional, aneuploidy, skeletal dysplasia, growth restriction

Pearls & Pitfalls

Adopt a consistent systematic sequence to avoid missing required views
Document all required cardiac views; the outflow tracts detect lesions missed on four-chamber alone
Report soft markers in the context of overall aneuploidy risk rather than in isolation
Reassess a low-lying placenta in the third trimester as most resolve
Fetal position and maternal habitus limit sensitivity; document incomplete views and offer a repeat scan
Confirm three-vessel cord and cord insertion site (rule out vasa previa if velamentous/low)
Beware pseudo-anomalies from off-axis planes; verify in orthogonal views

Structured Report

State indication, gestational age, fetal number, presentation, and image quality/limitations
Report biometry (BPD, HC, AC, FL) with percentiles and estimated fetal weight
Document systematic anatomy survey with normal or abnormal status of each system
Report placental location, cord vessel number, and amniotic fluid volume
Describe any anomaly or soft marker and its significance
Impression: growth, anatomy summary, and any concern with recommended follow-up/referral
Recommend fetal echocardiography, MFM referral, or repeat scan as appropriate

References

AIUM-ACR-ACOG-SRU Practice Parameter for the Performance of Detailed Second- and Third-Trimester Obstetric Ultrasound
ISUOG Practice Guidelines: mid-trimester fetal anatomy scan
Hadlock fetal biometry and growth reference standards
ACOG guidance on ultrasound in pregnancy and fetal anomaly detection
RCOG/NHS Fetal Anomaly Screening Programme standards

Educational clinical decision support only. Protocols vary by institution and equipment; always confirm with a qualified radiologist and local guidelines before clinical use.

Obstetric Ultrasound (Anatomy Scan)

📋 Indications

⛔ Contraindications & Cautions

🧑‍⚕️ Patient Preparation

⚙️ Technique & Parameters

🔎 Systematic Review

🩻 Key Findings & Significance

🧠 Differential Considerations

💡 Pearls & Pitfalls

📝 Structured Report

📚 References