Ultrasound Screening For Fetal Anomalies

Routine first trimester dating scanning has a number of benefits like
(1) it is more accurate at assessing gestational age than menstrual periods, and therefore reduces the
rates of induction of labour for post-term pregnancies
(2) detects multiple pregnancies early in pregnancy and
(3) can detect some major structural anomalies such as anencephaly.

Nuchal scan or Nuchal translucency (NT) scan: A  more detailed, although limited, anomaly scan is incorporated into a nuchal scan or nuchal translucency (NT) scan which helps in determining the basics such as

• the shape of the fetal skull, 
• presence of nose, hands and feet and 
• presence of stomach and bladder. 

Therefore, instigation of Down’s syndrome screening strategies such as the Combined or Integrated test which involve an NT scan are likely to not only increase the detection of aneuploidy but also major structural anomalies and particularly cardiac defects earlier in gestation.

Routine Second Trimester Ultrasound Screening For Fetal Anomalies 
In the UK and most of the other countries around the world. there is a policy of routine second trimester ultrasound screening for fetal anomalies .
However, detection of fetal anomalies varies considerably depending on the anomaly being screened for as well as the gestation at screening, the skill of the operator and the quality of the equipment used.

The detection of cardiac anomalies is of particular interest. Early prenatal detection of congenital heart disease (CHD) has increased due to advances in ultrasound resolution and the incorporation of at least a 4-chamber cardiac view in the routine anomaly scan.


Mid-trimester ultrasound markers
Soft-tissue markers are signs that can be detected on a second trimester anomaly scan that in themselves are not structural defects but have an association with aneuploidy, and therefore their presence increases the risk of aneuploidy.
Markers include

• nuchal skin oedema, 
• short femoral or humeral length measurements, 
• choroid plexus cysts, 
• bilateral renal pelvic dilatation, 
• echogenic fetal bowel and 
• hyperechogenic foci (‘golf balls’) in the fetal heart.

Most of the studies indicate that  only the presence of a thickened nuchal fold or echogenic bowel in isolation would significantly increase the risk of aneuploidy. However, the presence of any of the other soft markers would alert the person performing the scan to perform a thorough check for other features of aneuploidy. 

If indeed more than one soft marker or any structural anomaly is detected this would significantly increase the risk of aneuploidy and warrant further counselling regarding invasive karyotyping.