📋 What Is an Anomaly Scan?
An anomaly scan is a specialized ultrasound examination performed at 18–22 weeks of pregnancy to assess the fetal organs in detail. It is also known as a detailed ultrasound, level II ultrasound, fetal anatomy scan, or second-trimester scan.
Its purpose is to detect fetal structural anomalies and, if necessary, to plan early intervention. It is planned according to the current multi-society AIUM-ACR-ACOG-SMFM-SRU standard obstetric ultrasound guideline (2024) and the ISUOG (International Society of Ultrasound in Obstetrics and Gynecology) 2022 mid-trimester guideline.
⏰ Optimal Timing:
- 18-22 weeks: The interval when all fetal structures are best visualized
- 20-22 weeks: Most preferred, organ evaluation optimal
- After the 22nd week: May be preferred in mothers with high BMI (soft tissue increases)
🔍 What Is Evaluated During an Anomaly Scan?
According to ACOG and ISUOG, screening is done with a "checklist" approach. The following systems and structures are evaluated:
🧠 Central Nervous System
Head and Brain:
- Head shape (BPD, OFD, HC measurements)
- Lateral ventricles (atrium < 10 mm)
- Thalamus and cavum septum pellucidum
- Cerebellum (transcerebellar diameter)
- Cisterna magna (2-10 mm)
- Facial profile (nasal bone, lips)
❤️ Heart and Great Vessels
Fetal Heart Imaging:
- 4 chamber view (4CV)
- Left ventricular outflow tract (LVOT)
- Right ventricular outflow tract (RVOT)
- 3-vessel view (3VV)
- Aortic arch, ductus arteriosus
- Heart rhythm and rate (110-160 beats/min)
🫁 Thorax (Chest)
Lungs and Diaphragm:
- Echogenicity of two lungs
- Diaphragmatic integrity (CDH exclusion)
- Mediastinal structures
- Thorax/heart ratio
🫀 Abdomen
Intra-abdominal Organs:
- Stomach (upper left quadrant, fluid-filled)
- Liver and gallbladder
- Kidneys (both sides)
- Bladder (fluid-filled)
- Anterior abdominal wall (umbilical cord entry site, gastroschisis/omphalocele)
- Intestinal echogenicity
🦴 Skeletal System
Bones and Limbs:
- Vertebra (sagittal and axial) — spina bifida
- Upper and lower extremities (long bones, hands, feet)
- Fingers (5 fingers, polydactyly)
- Femur length (FL)
- Tibia, fibula, humerus, radius, ulna
- Skull shape and mineralization
👶 Genitals and Other
Sexual Organs and Joints:
- Gender (if requested)
- Genital structures
- Placenta placement (previa control)
- Placenta structure, thickness
- Umbilical cord (3 vessels — 2 arteries + 1 vein)
- Amniotic fluid amount (DVP, AFI)
📐 Biometric Measurements
During anomaly screening, the baby's growth is also evaluated. All measurements are compared to gestational age.
| Measurement |
Description |
Normal Range (20 weeks) |
| BPD |
Biparietal diameter (head diameter) |
~46mm |
| HC |
Head circumference |
~175mm |
| AC |
Abdominal circumference |
~150mm |
| FL |
Femur length |
~33mm |
| EFW |
Estimated fetal weight |
~320g |
| NF |
Nuchal translucency |
< 6 mm |
🚨 Soft Markers
Soft markers do not indicate anomalies on their own, but are used when calculating the risk for chromosomal anomalies. The current reference for isolated soft marker management in the cfDNA era is SMFM Consult Series #57 (2021, confirmed 2025); Diagnostic testing alone (amniocentesis) for an isolated marker after a negative serum/cfDNA screen is not recommended:
Common Isolated Soft Markers (SMFM Consult #57):
- Nasal bone absence/hypoplasia
- Hyperechogenic intestine
- Renal pyelectasis (renal pelvis ≥ 4 mm)
- Echogenic intracardiac focus (EIF) — of low significance if isolated
- Single umbilical artery (SUA)
- Choroid plexus cyst — not of clinical significance if isolated
- Short femur/humerus
📊 Screening Limitations
Anomaly Scan Cannot Find All Anomalies!
Even screening under ideal conditions can detect approximately 70-90% of all fetal anomalies. Some conditions are normal but occur after birth:
- Some cardiac anomalies (especially ASD, small VSD)
- Vision and hearing defects
- Developmental delays
- Microcephaly (may occur late)
- Late-onset hydrocephalus
Factors affecting scan quality: maternal obesity, oligohydramnios, fetal position, abdominal scars.
📌 What Happens in Case of Abnormal Finding?
- Confirmatory USG: Detailed 2nd examination, usually by a perinatologist.
- Fetal Echo: Fetal echocardiography in suspected cardiac anomaly.
- Invasive Testing Recommendation: Karyotype/CMA evaluation by amniocentesis or CVS.
- Multidisciplinary Council: Maternal-fetal medicine, pediatric surgery, genetics, neonatology.
- Family Information: Diagnosis, prognosis, next steps, support groups.
💡 Preparation and Practical Tips
- You do NOT need to go hungry (stomach fullness does not affect baby imaging)
- If you have previous USG records, bring them with you
- Examination may take 30-45 minutes, no full bladder required
- You can take someone you love with you
- If you do not want to know the gender, specify it in advance
- If the baby's position is inappropriate, you may be asked to walk or turn on your side