The anatomy scan finds about two thirds of what is there
In 40,335 low-risk US scans, 64% of major anomalies were seen on the initial scan. One in three was not.
The 18-to-22-week scan is the appointment most parents remember. It is also the one most often heard as a verdict on the whole baby.
The best figure for a US audience comes from a study of standard scans done at 18 to 21 weeks and 6 days in a low-risk population, with every baby checked after birth.
| Measure | Value |
|---|---|
| Standard scans performed | 40,335 |
| Major abnormalities confirmed in the babies | 387 (1%) |
| Found on the initial standard scan | 248, or 64% |
| Found on a first follow-up scan | 28 (7%) |
| Found on a second follow-up scan | 5 (1%) |
About two thirds. That is what the routine anatomy scan does in a low-risk pregnancy at a good US centre. It is a genuinely useful test, and it is not a guarantee.
Detection also depends heavily on which part of the body is being looked at. The classic breakdown comes from a large European study across 61 units, conducted between 1990 and 1993, so treat the exact percentages as a pattern rather than as today's numbers. Among major abnormalities, it found 88.3% of those in the central nervous system, 84.8% of those in the urinary tract, and 38.8% of those in the heart and great vessels.
While it is not possible to detect all structural congenital anomalies with diagnostic ultrasound, adherence to the following practice parameter will increase the likelihood of detecting many fetal abnormalities.
And on why:
some structures can be difficult to visualize because of fetal size, position, movement, abdominal scars, and increased maternal abdominal wall thickness.
Nothing in this topic is a criticism of ultrasound or of the person holding the probe. It is the same thing the professions have written down and signed.
"What could not be seen, and does it need another look?"
"When you say everything looks good, what does that cover and what does it not?"