Screening for chromosomal abnormalities

This article is also available in Italian.

We have already spoken of screening that are performed during the first trimester of pregnancy.

Now I’d like to talk about the tests that are done in the second trimester of pregnancy, to detect signs of so-called soft markers, which allow us to predict, to some extent, the possibility that our child is suffering from chromosomal defects.

You have already heard of soft markers. What are they?

These are structural features that are identified through ultrasound, which in itself is not a problem, but can be signs of a change in the number of chromosomes. The most common are:

• choroid plexus cysts,

• calcification of the cardiac papillary muscle (Echogenic intracardiac focus or “Golf Ball”),
• renal pelvis dilatation (pyelectasis),
• dilatation of the lateral cerebral ventricles,
• hyperechoic bowel.

These can be found in approximately 5% of all pregnancies in the second trimester and are often the cause of much anxiety for parents.

It is very difficult to determine exactly what impact these markers in chromosomal abnormalities, because we have not yet enough data to determine.

If we take them separately, the risk of chromosomal problems is low. But if there are more than one or there are other structural defects, the risk is very high.

We must not regard them as a screening test for chromosomal abnormalities because the majority of fetuses with Down syndrome have, in ultrasound, a normal appearance.

The best time to identify the soft markers is in ultrasound in the second trimester of pregnancy (between the 16th and 18th week).

Those listed above are not the only signs. We find also the following:

• intrauterine growth retardation is often associated with trisomy 18 (Edwards) and trisomy 13 (Patau) which, together with a quantity of amniotic fluid greater than 250 mm, will give us a very high predictive value.

• increased nuchal fold thickness, the threshold is 6 mm, is associated with Down syndrome, with a false positive rate of 3%. It is a thickening of the nuchal area due to an anomalous connection between the lymphatic system and nervous system. This marker, along with the nasal bone, are those with a higher predictive value.
• the absence or a reduced development of the region maxillary and nasal bone are other markers. It says that the nasal bone is absent or reduced development when it is smaller than 2.5 mm and is found in 61% of Down fetuses.
• the length of the humerus and femur are shorter than expected. To get an indication of the relative lengths in gestational age you can see the patterns that follow:
  • Femur length

  • Humerus length
• the asymmetry between the head and limbs, in the sense that the biparietal diameter is increased compared to the length of the limbs, especially humerus.

Technological development allows us to identify these signals, however, make anxious parents, often in vain, because most often disappear before birth without consequences.

Sources:

• A. Borrelli, D. Arduini, A. Cardone, V. Ventruto, “Medicina Dell’eta Prenatale”, Springer-Verlag Italia, 2008
• Ian A. Greer, Iain T. Cameron, Brian Magowan, “Problem-based obstetrics and gynaecology”, Churchill Livingstone, 2003
• Boccato, “Trattato italiano di medicina di laboratorio, Volume 8”, Piccin Nuova Libraria, 2006