Fetal Anomaly Screening Programme.

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Home What's in the hexagons 2.4 About the scan 2.4.2 Limitations of the scan

2.4.2 Limitations of the scan

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So they tell you it’s normal … But what does that really mean? Because the fact that they told me it’s normal, doesn’t mean that it is normal 100%. I could still have a baby with some problem. I know they are concepts that are a bit difficult to grasp …
[Pregnant woman]

Understanding the limitations

The scan is a screening test and therefore has limitations and there is a risk that some conditions will be missed or misidentified. [NHS FASP standards page 55]

It is essential that health professionals understand the limitations of the scan and the factors that may reduce the quality of the scan, so that they can prepare women as effectively as possible. As part of the consent process, women must be:

  • informed of the limitations of the scan and that it cannot identify all potential conditions or structural abnormalities.
  • advised of the factors that may impact adversely on the quality of the images obtained when they attend for their scan

[NHS FASP standard 5.3]

It is important that each woman comes to the scan with realistic expectations of the process and outcomes and that the issue of limitations is reinforced when sharing the outcomes of the scan with the woman. Unrealistic expectations may result in patient dissatisfaction with the service they have been offered and be a source of complaint (Health Care Commission 2009).

jane_stillClick here to listen to Jane Fisher, Director of ARC, reinforcing the importance of managing women’s expectation and ensuring women understand the limitations of the scan (QuickTime)

Possible limiting factors

Factor

Explanation

Illustrative images

Gestational age

Some conditions may not be detectable until later in pregnancy

Similarly some conditions may not manifest themselves at an earlier stage of pregnancy i.e. if the scan is undertaken too early

Click here to view image

Position of the fetus

The fetus may be lying awkwardly and this may restrict the views that can be obtained

Click here to view image

Body Mass Index (BMI)

A raised BMI reduces the way in which the sound waves travel and can significantly reduce the quality of the images generated.  Conversely it may be difficult to get good transducer contact and angulation in very thin women and those with toned abdominal muscles

Click here to view image

Presence of fibroids, scar tissue

May distort the sound waves and detract from the quality of the images generated

Click here to view image

Amniotic fluid volume

The volume of amniotic fluid, both increased and decreased, may make visualisation more difficult

Click here to view image

Multiple pregnancy

Multiple pregnancies can affect fetal position and cause shadowing which may reduce image quality. It may also be difficult to be certain which fetus is being examined.

Click here to view images

 

Repeat scan

If the image quality of the first 18+0‐20+6 weeks fetal anomaly screening scan is compromised by maternal factors and/or sub-optimal fetal positioning:

  • the woman should be offered  a single further scan in an attempt to complete the screening examination
  • the second examination should be performed before 23 completed weeks of pregnancy.

[NHS FASP standard 6.3]

Where adequate assessment of the fetal anatomy remains compromised after the second scan, the woman should be told that the screening is incomplete. This should be recorded. [NHS FASP standard 6.4]

trish_stillClick here to listen to Dr Trish Chudleigh, member of the National Standards Core Reference Group explaining the rationale for this decision to limit the number of repeat scans, particularly in relation to raised BMI (QuickTime)

 

Related standards

Click here to view related NHS FASP standards and guidelines

Video & Audio

Jane Fisher, Director of ARC reinforces the importance of managing women's expectations so that they understand the limitations of the scan. QuickTime

Dr Trish Chudleigh explains the rationale for limiting the number of repeat scans. QuickTime

UK NSC: What is screening?

Other Relevant Sections

2.2.3 NHS FASP consent standards

2.3.1 Knowing about the woman before the scan

2.3.2 Preparing women for the scan

2.3.4 Sharing the outcomes of the scan

Activities

Activity 1: Limitations of the scan
For midwives