20 Week Anomaly Scan: Complete Anatomy Survey Guide

The 20-week anomaly scan, formally called the second-trimester anatomy survey or morphology scan, is a comprehensive ultrasound examination performed between 18-22 weeks gestation to systematically assess fetal anatomy and detect structural abnormalities. The examination evaluates multiple organ systems including the brain and spine (neurosonography), face, heart (four-chamber view and outflow tracts), abdominal wall, stomach, kidneys, bladder, limbs (arms and legs, hands and feet), and placental location. Current evidence indicates that the anomaly scan detects approximately 50-70% of major fetal anomalies, with detection rates varying significantly by specific condition and factors such as maternal body habitus, fetal position, gestational age, amniotic fluid volume, and operator expertise. The examination typically takes 30-60 minutes and may require a full bladder depending on gestational age and maternal factors. Limitations include inability to detect all abnormalities, some conditions that develop later in pregnancy, and technical factors that limit visualization. Abnormal findings may be managed with repeat imaging, referral to fetal medicine specialists, additional testing, or in some cases, pregnancy termination planning. This comprehensive guide details the anatomical structures assessed, examination timing, detection rates, limitations, follow-up for abnormal findings, and what expectant parents should know about this important screening examination.

Purpose and Goals

The primary purpose of the 20-week anomaly scan is to screen for structural fetal abnormalities through systematic assessment of all major organ systems. The examination has multiple goals:

Primary Goals:

• Structural assessment: Systematically evaluate all major fetal organ systems
• Anomaly detection: Identify major structural abnormalities that may affect management
• Growth assessment: Evaluate fetal growth and amniotic fluid volume
• Placental assessment: Determine placental location and rule out placenta previa
• Confirmation: Confirm gestational age and number of fetuses

Secondary Goals:

• Incidental findings: Identify other findings that may affect pregnancy management
• Baseline assessment: Establish baseline anatomy for comparison with subsequent scans if needed
• Reassurance: Provide reassurance if anatomy appears normal
• Management planning: Allow for planning and coordination of care if abnormalities are identified

The examination is considered a standard component of routine prenatal care and is recommended by professional organizations including the American College of Obstetricians and Gynecologists (ACOG) and the American Institute of Ultrasound in Medicine (AIUM).

What Makes the 20-Week Scan Different

The 20-week scan differs from other pregnancy ultrasounds in several important ways:

The 20-week scan represents the most detailed and comprehensive anatomical assessment performed during routine pregnancy, with systematic evaluation of all major organ systems rather than focusing on a specific concern.

Required Examination Components

Professional organizations have established standardized components that should be assessed during the anatomy scan:

1. Fetal Number and Presentation

• Confirm singleton or multiple gestation
• Document fetal presentation (cephalic, breech, transverse)

2. Cardiac Activity

• Confirm presence of cardiac activity
• Document heart rate (typically 120-160 bpm)

3. Biometric Measurements

• Biparietal diameter (BPD): Head width
• Head circumference (HC): Head size
• Abdominal circumference (AC): Abdominal size
• Femur length (FL): Thigh bone length
• Used to confirm gestational age and assess growth

4. Detailed Anatomical Assessment

Cranium and Brain:

• Cranial shape and integrity (exclude neural tube defects)
• Lateral ventricles (assess for ventriculomegaly)
• Choroid plexus (assess for choroid plexus cysts)
• Cerebellum and cisterna magna (assess for posterior fossa abnormalities)
• Midline falx (assess midline integrity)

Face:

• Profile (exclude facial bone abnormalities)
• orbits (eye sockets)
• Upper lip (exclude cleft lip)
• Nasal bone

Neck:

• Nuchal region (assess for cystic hygroma or other masses)

Spine:

• Cervical, thoracic, lumbar, and sacral vertebrae
• Skin covering over spine (exclude spina bifida)
• Spinal curvature (exclude scoliosis)

Thorax:

• Lung appearance
• Cardiac position and situs

Heart:

• Four-chamber view (assess chamber size and symmetry)
• Ventricular septum (exclude VSD)
• Atrial septum (exclude ASD)
• Atrioventricular valves
• Left and right ventricular outflow tracts
• Three-vessel view

Abdomen:

• Stomach (presence in left upper quadrant)
• Liver
• Umbilical cord insertion (exclude abdominal wall defects)
• Abdominal wall integrity

Kidneys and Urinary Tract:

• Both kidneys present
• Renal pelvis (assess for hydronephrosis)
• Bladder (visible when filled)

Limbs:

• All four limbs present
• Arms: humerus, radius, ulna
• Legs: femur, tibia, fibula
• Hands and feet
• Fingers and toes (number and appearance)

Placenta and Cord:

• Placental location (exclude previa)
• Placental appearance
• Umbilical cord vessels (normally 3 vessels: 2 arteries, 1 vein)
• Cord insertion site

Amniotic Fluid:

• Subjective assessment of volume
• Maximum vertical pocket measurement (if indicated)

Optional/Extended Assessment

Depending on facility protocol, gestational age, and visualization, the following may also be assessed:

• Detailed cardiac echocardiography: Detailed evaluation of cardiac structures and function
• Facial features: Detailed facial assessment including nose, lips, and ears
• Extremity detail: More detailed assessment of hands, feet, fingers, and toes
• Cervical length: Assessment of cervical length in patients at risk for preterm birth
• Uterine anatomy: Assessment for uterine abnormalities or fibroids

Optimal Gestational Age

The optimal timing for the anatomy scan balances multiple factors:

Recommended Timing: 18-22 weeks

The 18-22 week window is recommended because:

• Fetal size is adequate for detailed anatomical assessment
• Organs are developed enough to assess structure and position
• Fetal position can often be changed by having mother move around
• Amniotic fluid is typically adequate for visualization
• There is still time for additional testing, consultations, and decision-making if abnormalities are detected
• Many structural abnormalities are detectable by this gestational age

Factors Affecting Timing

Several factors may affect optimal timing for individual patients:

Factors Favoring Earlier Scan (18-19 weeks):

• Multiple gestation (becomes more crowded earlier)
• Poor visualization anticipated (high maternal BMI, anterior placenta)
• Previous pregnancy with structural abnormality
• Patient anxiety and desire for earlier information
• Need more time for follow-up testing if needed

Factors Favoring Later Scan (21-22 weeks):

• Improved visualization of certain structures (heart, great vessels)
• More developed fetal features
• Previous early scan with limited views

Overall Detection Rates

The overall detection rate for major structural abnormalities with the 20-week anatomy scan is approximately 50-70%, with significant variation by specific abnormality and multiple factors affecting detection:

Factors Affecting Detection

Multiple factors affect the ability to detect abnormalities:

Patient Factors:

• Maternal body habitus: Detection rates decreased with maternal BMI > 30, further decreased with BMI > 35
• Abdominal wall scars: Previous surgeries may impair visualization
• Uterine position: Retroverted uterus may affect visualization
• Fetal position: Suboptimal fetal position prevents complete assessment

Pregnancy Factors:

• Gestational age: Earlier or later than optimal window reduces detection
• Amniotic fluid volume: Oligohydramnios impairs visualization; polyhydramnios may also reduce quality
• Multiple gestation: More difficult to assess each fetus completely
• Placental location: Anterior placenta may obstruct visualization

Technical Factors:

• Ultrasound equipment: Higher-end equipment with better resolution improves detection
• Operator experience: More experienced operators detect more abnormalities
• Examination duration: Rushed examinations are more likely to miss abnormalities
• Fetal activity: Active fetus may be more difficult to assess systematically

Immediate Steps

When a potential abnormality is detected during the anatomy scan:

1. Confirmation attempt: The sonographer typically has a colleague or radiologist confirm the finding
2. Documentation: Images are obtained for documentation and possible review
3. Referral: Referral to a maternal-fetal medicine specialist or tertiary center for specialized evaluation
4. Counseling: Genetic counseling and counseling regarding the specific finding
5. Additional testing: May offer diagnostic testing (amniocentesis) if not already performed

Management Based on Finding Type

Management depends on the specific abnormality detected:

Minor or Likely Benign Findings:

• Choroid plexus cysts: Usually resolve spontaneously; if isolated and no other markers, typically considered benign
• Mild pyelectasis: Slightly dilated renal pelvis; if isolated and mild, typically resolves
• Echogenic intracardiac focus: Small bright spot in heart; if isolated, typically benign variant
• Short femur/humerus: If only mildly short and no other findings, may be normal variant

Major Structural Abnormalities:

• Neural tube defects: Immediate referral to maternal-fetal medicine, possible MRI, discussion of prognosis, delivery planning
• Cardiac defects: Referral to pediatric cardiology, fetal echocardiography, delivery planning at appropriate facility
• Abdominal wall defects: Referral to pediatric surgery, delivery planning, possible in-utero transport
• Limb abnormalities: Referral to genetics and orthopedics, further evaluation

Chromosomal Abnormality Markers:

• Multiple soft markers: Considered higher risk; offer amniocentesis for definitive diagnosis
• Single soft marker: Risk depends on specific marker and patient age; individualized counseling

Outcomes

Outcomes vary widely depending on the specific abnormality:

Important Limitations

Understanding the limitations of the anatomy scan is crucial:

Detection Limitations:

• Not 100% detection: 30-50% of major structural abnormalities are missed
• Cannot detect all conditions: Many conditions cannot be detected prenatally
• Normal scan ≠ perfect baby: A normal anatomy scan significantly reduces but does not eliminate risk for abnormalities
• Some conditions develop later: Some conditions (e.g., bowel obstruction, some cardiac problems) develop later in pregnancy

Technical Limitations:

• Visualization challenges: Not all structures can be visualized in all patients
• Fetal position: Fetal position may prevent adequate visualization of certain structures
• Maternal factors: Maternal body habitus, abdominal wall scars, and uterine position may impair visualization

Conditions Not Detectable by Anatomy Scan:

• Most genetic/metabolic disorders: Conditions like cystic fibrosis, metabolic disorders typically not detectable
• Many chromosomal abnormalities: Most chromosomally abnormal fetuses appear structurally normal
• Neurodevelopmental disorders: Autism, intellectual disability, cerebral palsy not detectable
• Hearing or vision problems: Typically not detectable on prenatal ultrasound
• Minor abnormalities: Many minor abnormalities not detectable or not visible at 20 weeks

Preparation

Preparation for the anatomy scan typically includes:

• Gestational age timing: Schedule between 18-22 weeks, ideally 20-21 weeks
• Full bladder: May be requested depending on gestational age and maternal factors; drink 24-32 ounces of water 1 hour before
• Previous records: Bring any previous ultrasound records or reports
• Glucose intake: Some facilities recommend having a snack or drink before to encourage fetal activity
• Clothing: Wear loose, comfortable clothing with easy access to abdomen
• Duration: Plan for 30-60 minutes for the examination

During the Examination

The examination typically proceeds as follows:

1. Initial assessment: Confirm fetal number, position, and heart rate
2. Biometric measurements: Measure BPD, HC, AC, FL to confirm dating and assess growth
3. Systematic anatomy survey: Examine each organ system systematically
4. Documentation: Obtain images of required structures
5. Position changes: May ask you to move around, empty bladder, or change position to improve visualization
6. Real-time assessment: Observe fetal movement and behavior
7. Final review: Review images to ensure all required structures have been adequately assessed

Possible Outcomes at Completion:

• Complete normal examination: All required structures seen and appear normal
• Complete with minor findings: Some minor or likely benign findings identified
• Incomplete examination: Some structures not well visualized; may recommend follow-up scan
• Abnormal findings: Potential abnormality identified; referral for further evaluation

Is the 20-week anatomy scan required or optional? The anatomy scan is considered a standard component of routine prenatal care and is recommended by professional organizations including ACOG. However, as with all medical procedures, patients have the right to decline. Most patients choose to have the anatomy scan because it provides important information about fetal anatomy and can detect many serious abnormalities that may affect management and delivery planning.

What if they can't see everything during my anatomy scan? In approximately 10-20% of cases, not all required structures can be adequately visualized due to fetal position, maternal body habitus, or other factors. In these cases, the report will indicate which structures were not well visualized, and a follow-up scan in 2-4 weeks may be recommended. This is common and not a cause for alarm; the fetus often moves into better position for the follow-up examination.

Does a normal anatomy scan guarantee my baby will be healthy? No, a normal anatomy scan does NOT guarantee a healthy baby. The scan detects approximately 50-70% of major structural abnormalities, meaning 30-50% are missed. Many conditions cannot be detected prenatally, including most genetic disorders, chromosomal abnormalities (without structural findings), neurodevelopmental disorders, and many minor abnormalities. A normal scan is reassuring but does not eliminate all risks.

Can the anatomy scan detect Down syndrome? The anatomy scan can detect certain markers that may be associated with Down syndrome (so-called "soft markers" including increased nuchal fold, echogenic intracardiac focus, choroid plexus cysts, pyelectasis, and short humerus/femur). However, most fetuses with Down syndrome have a structurally normal anatomy scan. The anatomy scan is NOT a reliable screening test for Down syndrome; first-trimester screening (NT scan + blood work) or NIPT are the appropriate screening tests for Down syndrome.

What happens if an abnormality is found? If an abnormality is identified, you will be referred to a maternal-fetal medicine specialist (high-risk obstetrician) for specialized evaluation. This may include repeat detailed ultrasound, fetal echocardiogram (for heart abnormalities), or fetal MRI (for certain abnormalities). Genetic counseling and diagnostic testing (amniocentesis) may be offered. The management depends on the specific abnormality but may include pregnancy termination planning, delivery at a tertiary center, or planning for immediate surgery after birth.

• The 20-week anomaly scan is a comprehensive anatomical survey performed between 18-22 weeks that systematically evaluates all major fetal organ systems and detects approximately 50-70% of major structural abnormalities.

• Required examination components include assessment of the brain and spine, face, heart (including four-chamber view and outflow tracts), abdominal organs and wall, kidneys and bladder, all limbs and extremities, and placental location.

• Detection rates vary widely by specific condition, from >95% for anencephaly to ＜15% for isolated cleft palate, with cardiac abnormalities showing particularly variable detection rates of 30-60%.

• A normal anatomy scan is reassuring but does NOT guarantee a healthy baby, as 30-50% of major structural abnormalities and most genetic, chromosomal, and neurodevelopmental conditions cannot be detected.

• Limited visualization is common, occurring in 10-20% of cases due to fetal position, maternal body habitus, or other factors, typically requiring follow-up scan in 2-4 weeks.

• Abnormal findings are managed with referral to maternal-fetal medicine specialists, additional imaging and testing, genetic counseling, and individualized management planning based on the specific abnormality.

• The examination takes 30-60 minutes, may require a full bladder, and involves systematic assessment of multiple organ systems with position changes and patient cooperation to optimize visualization.

• Professional organizations recommend the anatomy scan as a standard component of routine prenatal care, though patients have the right to decline after understanding the benefits and limitations.