Body size changes what the scan can see
After a normal scan, the chance a major anomaly is still there runs from about 4 in 1,000 to about 10 in 1,000.
This topic exists because the information is routinely left out, and leaving it out is not kind. It is simply less accurate counselling for some women than for others.
A study of 10,112 standard scans and 1,098 detailed scans at 18 to 24 weeks measured how detection changed with maternal body mass index.
| Maternal BMI category | Detection, standard scan | Detection, detailed scan |
|---|---|---|
| Normal BMI | 66% | 97% |
| Overweight | 49% | 91% |
| Class I obesity | 48% | 75% |
| Class II obesity | 42% | 88% |
| Class III obesity | 25% | 75% |
The single most useful number in that paper is not in the table. It is the residual risk: the chance that a major anomaly is still present after a normal scan. That figure rose from 0.4% among women of normal BMI to 1.0% among women with obesity. In whole numbers, roughly 4 in 1,000 versus roughly 10 in 1,000. Those are absolute figures taken straight from the study.
The same study found detection was also lower in pregnancies affected by diabetes that predated the pregnancy, at 38% compared with 88% in other high-risk indications. The authors' conclusion was blunt: Counseling may need to be modified to reflect the limitations of ultrasonography in obese women.
The joint professional standard already lists
increased maternal abdominal wall thicknessamong the things that make structures hard to see. Saying it out loud is not a judgement about anyone's body. Not saying it is a decision to let one group of women be less well informed than another.
"Given my body, would a detailed scan see more than a standard one?"
"After a normal scan, what is the chance in my case that something is still there?"