An anomaly scan in pregnancy is a detailed mid-pregnancy ultrasound, usually performed between 18 and 22 weeks of gestation, that examines the baby's anatomy for structural abnormalities and confirms growth, placental position, and amniotic fluid volume. It is offered to essentially all pregnant women in the United States and Europe, takes 30 to 45 minutes, and detects roughly 50 to 75 percent of major structural anomalies depending on the condition, the operator, and maternal body habitus. It is a screening test, not a diagnosis: a normal scan lowers but never eliminates the chance of a birth defect, and an abnormal finding almost always requires follow-up imaging or genetic testing.
What the Anomaly Scan Actually Measures
The scan is a systematic anatomical survey, not a single picture. Sonographers follow a checklist of roughly 20 to 30 views defined by professional bodies such as the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) and, in the US, the American Institute of Ultrasound in Medicine.
The operator measures the head circumference, biparietal diameter, abdominal circumference, and femur length, then combines them into an estimated fetal weight. These four measurements, taken together, predict gestational age within about 7 to 10 days in the second trimester. Crown-rump length measured at 11 to 14 weeks remains the most accurate dating method; the anomaly scan refines but does not replace it.
The checklist then walks through organs in sequence: skull shape and brain structures (cavum septum pellucidum, cerebellum, ventricles), face and palate, four-chamber heart and outflow tracts, spine in three planes, stomach, kidneys, bladder, abdominal wall, and all four limbs with hands and feet. The placental site, cord insertion, and deepest vertical pocket of amniotic fluid are recorded.
The mechanism matters because the scan depends on acoustic windows. Bone blocks sound, so the skull, spine, and ribs are hard to see through late in pregnancy. Fat and scarring in the maternal abdominal wall scatter the beam. This is why the scan is timed to a window where organs are large enough to resolve but bones are not yet densely ossified. In plain terms: the scan works best at a specific size, not a specific age.
Anomaly Scan Timeline: When Each Structure Is Best Seen
The 18 to 22 week window is a compromise, and different structures peak at different times. The table below shows why the timing is what it is.
| Gestational age | What is best assessed | Key threshold or note |
|---|---|---|
| 11–14 weeks | Nuchal translucency, nasal bone, early gross anatomy | NT above 3.5 mm flags increased risk |
| 15–18 weeks | Kidneys, bladder, abdominal wall often still small | Too early for reliable cardiac outflow views |
| 18–20 weeks | Full anatomical survey, cardiac four-chamber view | Standard lower bound for the anomaly scan |
| 20–22 weeks | Optimal balance of organ size and bone shadowing | Preferred window for most protocols |
| 22–24 weeks | Late anatomy, growth, placental position | Still acceptable; some organs harder to see |
| 24–28 weeks | Growth surveillance, cervical length if indicated | Not a substitute for the anomaly scan |
| 32–36 weeks | Growth, presentation, placental location | Detects late-onset conditions only |
The narrow sweet spot is around 20 weeks. Before 18 weeks, the fetal heart is roughly the size of a grape and outflow tracts are often unreadable. After 22 weeks, the skull and spine cast acoustic shadows that obscure the brain and posterior fossa. A 2019 analysis in Ultrasound in Obstetrics & Gynecology found that detection rates for cardiac defects climb steeply between 13 and 22 weeks and then plateau.
Detection Rates by Condition: A Quantified Comparison
A common misconception is that the anomaly scan either finds a problem or clears the baby. Reality is graded, and the grade varies enormously by condition.
| Condition | Approximate detection rate at 18–22 weeks | Why the gap exists |
|---|---|---|
| Anencephaly | Over 95% | Absent skull vault is unmistakable |
| Spina bifida (open) | 80–90% | Requires good spine views; closed defects lower |
| Cleft lip | 70–90% | Palate-only clefts often missed |
| Congenital heart defects (all) | 40–60% | Outflow tract defects need color Doppler |
| Hypoplastic left heart | 70–85% | Four-chamber view usually abnormal |
| Gastroschisis | Over 90% | Bowel visible outside the abdominal wall |
| Renal agenesis (one kidney) | 60–80% | Depends on fluid and fetal position |
| Down syndrome (Trisomy 21) | 50–70% via soft markers alone | Markers are nonspecific; cfDNA is better |
The cardiac numbers deserve emphasis. Congenital heart disease affects roughly 8 to 9 per 1,000 live births, making it the most common major birth defect, and the four-chamber view alone misses many outflow tract anomalies. Per the 2023 ISUOG cardiac screening guidelines, adding outflow tract views raises detection to roughly 60 to 70 percent, still short of the near-100 percent achieved by fetal echocardiography in expert hands.
A second quantified point: a 2021 meta-analysis of 12 studies found that adding a mid-trimester scan to routine care reduced perinatal mortality by about 1 in 1,000 births, a small but real effect driven mostly by earlier delivery planning for lethal anomalies.
Why the Scan Is a Screening Tool, Not a Diagnosis
The anomaly scan's job is to sort pregnancies into lower- and higher-risk groups. When it finds something, the next step is targeted imaging, often a fetal echocardiogram or a level II ultrasound at a maternal-fetal medicine center, plus genetic counseling and possibly diagnostic testing such as amniocentesis or chorionic villus sampling.
Cell-free DNA screening has changed this calculus. Since 2011, cfDNA testing has detected Trisomy 21 with sensitivity above 99 percent from a maternal blood draw at 10 weeks or later. It does not replace the anomaly scan, because cfDNA cannot see a cleft palate or a heart defect. The two tests answer different questions, and both belong in modern prenatal care.
The trade-off is anxiety. A soft marker such as an echogenic intracardiac focus appears in about 3 to 5 percent of normal pregnancies. Isolated, it raises Down syndrome risk only modestly, but it triggers follow-up and worry. Professional societies now advise against treating an isolated marker as diagnostic.
Accuracy Limits and Edge Cases
Detection depends on more variables than most patients realize. Maternal body mass index above 30 reduces image quality and can drop cardiac detection by 10 to 20 percentage points. Anterior placenta, oligohydramnios below a deepest vertical pocket of 2 cm, and fetal position (breech, spine-up) all obscure views. Sonographer experience is the single largest operator variable; a 2018 study found detection rates for the same defects varied from 35 to 85 percent across centers.
There is also a timing edge case: a normal 20-week scan cannot rule out conditions that appear later. Hydrocephalus, some cardiac defects, and bowel obstructions can develop or become visible only in the third trimester. A repeat growth scan at 32 to 36 weeks is standard for this reason.
One corrected misconception: the anomaly scan does not measure the baby's sex for medical reasons in most protocols. Genital views are incidental, and some patients decline them.
What to Actually Do Around Your Scan
Prepare practically. Book the scan between 20 and 22 weeks if your provider offers flexibility. Wear two-piece clothing for abdominal access. Empty your bladder unless told otherwise, since a full bladder can distort the lower uterine segment. Bring your prior scan reports; the sonographer needs the dating scan to compare growth.
Ask three questions at the visit: Was every structure on the checklist seen? If not, what was missed and why? Is a repeat scan recommended? And if a finding appears, what is the next test and how soon? Request the written report, not just a verbal summary.
If a concerning finding appears, do not act on a single scan. Ask for a referral to maternal-fetal medicine and a fetal echocardiogram within one to two weeks. Time matters for decisions about delivery planning, but most anomalies are compatible with a normal pregnancy outcome.
Red Flags to Raise Immediately
- Any finding of absent or reversed end-diastolic flow on umbilical artery Doppler.
- A deepest vertical pocket below 2 cm (oligohydramnios) or above 8 cm (polyhydramnios).
- A short cervix under 25 mm at 18 to 24 weeks in a patient with prior preterm birth.
- Placenta covering the internal cervical os at 20 weeks (may resolve; needs repeat at 32 weeks).
- Estimated fetal weight below the 10th percentile with abnormal Dopplers.
- Any cardiac finding involving a single ventricle or absent outflow tract.
What the Scan Cannot Tell You
The anomaly scan is a snapshot, not a prediction of intelligence, future health, or behavior. It cannot detect most metabolic disorders, many genetic syndromes with normal anatomy, or hearing loss. It also cannot confirm a normal outcome for certain; a 20-week normal result still leaves roughly a 3 to 5 percent chance of a birth defect discovered later. The scan is powerful because it changes management, not because it provides certainty. That distinction, more than detection rates, is what patients and clinicians should carry into the appointment.