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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.