Anomaly Scan and Level II Imaging
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Direct answer
The anomaly scan is a detailed second-trimester ultrasound, usually performed at 18 to 22 weeks, that examines fetal structures for major malformations. Level II imaging is a more detailed examination by a specialist with advanced equipment, used when the routine scan is abnormal or risk factors are present. Detection rates vary with the organ system, fetal position and maternal body habitus.
What you must remember
- Timing: Usually 18 to 22 weeks, when most organs are visible and fetal growth can be assessed.
- Brain: Lateral ventricles, cerebellum and cisterna magna; an atrial width of 10 mm or more suggests ventriculomegaly.
- Heart: Four-chamber view and outflow tracts, since cardiac defects are among the most frequently missed anomalies.
- Abdomen and kidneys: Stomach bubble, kidneys, bladder and the abdominal wall, including the umbilical insertion.
- Spine and limbs: Spine, long bones, hands and feet, with attention to skeletal shape and number of digits.
- Placenta and cord: Placental location, grade and the number of umbilical cord vessels; a single umbilical artery prompts a search for other anomalies.
- Limits and disclosure: Maternal obesity, low liquor and unfavourable position reduce detection, and fetal sex must never be disclosed under the PCPNDT Act.
Structures examined by organ system
The scan follows a fixed sequence so that each organ system is documented, which reduces missed findings. The table lists the main structures and the concerns they raise.
| System | Key structures | Typical concern |
|---|---|---|
| Brain | Ventricles, cerebellum, cisterna magna | Ventriculomegaly, posterior fossa change |
| Heart | Four-chamber view, outflow tracts | Septal and outflow defects |
| Abdomen | Stomach, kidneys, bladder, abdominal wall | Absent stomach bubble, renal anomaly, omphalocele |
| Spine and limbs | Spine, long bones, hands and feet | Spinal defects, skeletal dysplasia |
| Placenta and cord | Location, grade, three vessels | Placenta praevia, single umbilical artery |
Level II imaging adds targeted views, colour Doppler and fetal echocardiography when a specific anomaly is suspected. It is the usual route for families with a family history or an abnormal routine scan.
Level II imaging is ideally performed in a unit with a specialist fetal medicine team, because findings often need immediate multidisciplinary discussion. Images and measurements are archived so that any change can be compared on a later scan.
Where the exam tries to catch you
Questions test the timing window, the limits of detection and the legal restriction on sex disclosure.
- Trap: Assuming a normal anomaly scan excludes all malformations. Correct: Detection is incomplete, and subtle cardiac defects and skeletal dysplasias are often missed.
- Trap: Telling the family the fetal sex during the scan. Correct: Disclosure of sex is prohibited under the PCPNDT Act, so the sonologist must not reveal it.
- Trap: Treating an isolated single umbilical artery as harmless. Correct: It is associated with other anomalies, so a detailed scan and fetal echocardiography are advised.
- Trap: Doing the structural survey before 18 weeks. Correct: Most structural anomalies are assessed in the second-trimester window, so the full anomaly survey is done at 18 to 22 weeks.
Frequently asked questions
At what gestational age is the anomaly scan performed?
The anomaly scan is usually performed between 18 and 22 weeks of gestation, when most organ systems are well visualised. Scans much earlier may miss structural problems. Scans later in pregnancy leave less time for diagnosis and counselling.
What is the difference between a routine anomaly scan and level II imaging?
A routine anomaly scan screens all pregnancies for major malformations, while level II imaging is a more detailed specialist examination. Level II imaging is used when a routine scan is abnormal or when risk factors such as family history exist. Fetal echocardiography may be added at that stage.
Which structures are examined in an anomaly scan?
The anomaly scan examines the brain, face, neck, heart, abdomen, kidneys, spine, limbs and placenta, along with the number of umbilical cord vessels. It also assesses fetal growth and amniotic fluid volume. Each organ system is documented to reduce missed findings.
Why do anomaly scans miss some malformations?
Anomaly scans can miss malformations because of maternal obesity, low amniotic fluid, unfavourable fetal position and the subtlety of some defects. Cardiac and skeletal anomalies are particularly prone to being missed. Detection therefore depends on operator skill and equipment.