Nuchal Translucency Assessment

At Broadway Radiology we offer a pregnancy ultrasound scan between 11 and 14 weeks gestation to measure the nuchal translucency of the foetus.  A nuchal translucency scan is a non-invasive test performed to assess the risk of a foetus having a chromosomal abnormality, such as Downs Syndrome (Trisomy 21).  It is sometimes called the “nuchal scan” or “nuchal screening” or “nuchal fold assessment”. 

Who should have a Nuchal Translucency Test?

Women undergoing an ultrasound for nuchal translucency assessment require a referral from their Lead Maternity Carer, and must be between 11 and 14 weeks gestation (12 weeks is the optimal time).  It is often most valuable for women of increased maternal age, and for women who have a family history of foetal abnormalities, or who have had a previous baby with an abnormality.   Women who meet these criteria are known to carry an increased risk of delivering a baby which a chromosomal abnormality.

Assessment of Risk:

During ultrasound scanning, a measurement is taken of the fluid in the skin fold at the back of the neck of the foetus.  Studies have shown that a greater than normal amount of fluid has been associated with an increased risk of a chromosomal abnormality.  The nuchal translucency measurement along with the mother’s age is used to estimate the specific risk of the foetus having a chromosomal abnormality.

The risk for Down Syndrome in Relation to Maternal Age:

Maternal Age (Years)	Risk for Trisomy 21
	At Birth	At 12 weeks
20	1 in 1527	1 in 898
25	1 in 1352	1 in 795
30	1 in 895	1 in 526
32	1 in 659	1 in 388
34	1 in 446	1 in 262
36	1 in 280	1 in 165
38	1 in 167	1 in 98
40	1 in 97	1 in 57
42	1 in 55	1 in 32
44	1 in 30	1 in 18

This table shows how the risk of delivering a baby with a chromosomal abnormality increases with maternal age. (In the first trimester of pregnancy, the risk that the foetus has a chromosomal abnormality is higher than at birth, because many affected foetuses die naturally during the pregnancy).

Further Testing:

It is important to understand that the nuchal translucency scan gives an estimate of the risk of a foetus having an abnormality.  It does not diagnose or exclude the presence of an abnormality.  To be certain whether or not a foetus has a chromosomal abnormality would require an invasive test such as an amniocentesis or Chorionic Villus Sampling (CVS).  Nuchal translucency is useful to determine whether further investigation is indicated.  If the risk is greater than 1 in 300 further testing may be offered.  If necessary, your Lead Maternity Carer may refer you to a Specialist Obstetrician for consideration of further procedures. 

Amniocentesis is usually performed at 15-17 weeks and involves passing a thin needle into the uterus to extract some fluid from around the foetus for testing.  CVS is performed at 11-12 weeks where some cells from the placenta are removed by a fine needle passed through the mothers’ abdomen for examination.

You may be offered maternal serum screening (triple test) at 16 weeks gestation.  This will give you an alternative non-invasive estimate of the risk for Downs Syndrome based on biochemical markers in your blood.

The nuchal translucency scan may also assist in the early diagnosis of any major anatomical defect, although it is important to have a routine anatomy scan at 19-20 weeks for a full evaluation of the foetal anatomy. 

 How Successful is Nuchal Translucency Screening?

Worldwide studies have shown that of women undergoing nuchal translucency screening, more than 80% of foetuses with chromosomal abnormalities have been identified.  This detection rate compares favourably with those achieved with screening based on maternal age (40%) or maternal age and serum biochemistry (65%).

Palmerston North X-Ray
Broadway Radiology, 197 Broadway Avenue, P.O. Box 8, Palmerston North.
Telephone 06 357-9079  Facsimile 06 357-9094