Maternal age can no longer be regarded as the preferred screening method for fetal trisomy 21 because of the ever increasing maternal age as well as the low detection rate (around 30%). With screening by maternal age the yield of invasive testing is low (i.e. a large number of procedures need to be done to find one abnormal result).

Of all the screening tests for trisomy 21, nuchal translucency (NT) measurement by ultrasound between 11 and 14 weeks gestation is currently the most sensitive single screening parameter. Measured by a strictly defined technique, NT is used in combination with the mother's age and previous history to calculate her individual risk for fetal trisomy 21. NT-screening has a detection rate for trisomy 21 of approximately 75% for a 5% false positive rate.

NT can be used in combination with multiple maternal serum parameters (beta-HCG, PAPP-a) or other ultrasound parameters (e.g. visibility of the nasal bone or the flow pattern in the venous duct) to increase detection rates  (to around 90%) without increasing the number of invasive procedures.

It is clear that the best screening test on offer for expectant parents is NT-measurement, either alone or in combination. However, it has been recognised that NT risk assessment relies on a standardised technique. Without this, NT-detection rates for T21 are disappointingly low (15-30%) because small differences in NT measurements due to subtle differences in technique, translate into major inaccuracies in risk assessment. The Fetal Medicine Foundation, UK, has developed a software program for NT risk assessment as well as a training program for correct NT-measuring technique. This training program has been widely adopted throughout the world and is internationally regarded as the gold standard for ultrasound screening. SASUOG fully supports the FMF-program as the ONLY accurate NT-screening program available and acknowledges the poor performance of NT-screening outside this program. Most South African laboratories  offering first trimester serum screening support this program as well, in view of the medicolegal implications if a proven inaccurate screening method was used instead.

Where are we now with NT-screening in South Africa?

We have had 3 theoretical courses on NT-screening so far (Cape Town and Pretoria (1998), Midrand (2002)). It was a great pleasure to hear the founders of the FMF-program, Professor Kypros Nicolaides and Ms Rosalinda Snijders, explain the background to NT-screening and the technical issues required to make NT-screening successful.

So far, more than 300 obstetricians / sonographers in South Africa, have attended this theoretical course. SASUOG is absolutely thrilled that a daily increasing number of them have completed the FMF training program and, to this date, more than 40 are accredited by the FMF, UK. Those doctors will have the reassurance that the T21-risk they quote to their patients is a true reflection of the actual risk and not an inaccurate estimate. Patients will have the reassurance that their decision regarding invasive testing is based on an honest and accurate risk assessment. Accredited practices are now available in most major centres in the country: Bloemfontein, Cape Town, Durban, Johannesburg, Houghton, Irene, Lenasia, Pietermaritzburg, Pietersburg, Pretoria, Richards Bay, South Rand, Vanderbylpark, Westville. Audit of how the NT-screening program is working in South Africa will be carried out by the end of the year.

Interested in training or accreditation? Contact Lut Geerts at:

PoBox 19081, Tygerberg 7505

Fax: 021 9316595

e-mail: lgeerts@sun.ac.za

As an appetizer for those not yet registered and, as a reminder for the accredited doctors, we enclose the requirements for successful NT-screening.

1. Good quality equipment:

Transabdominal probe is sufficient in 95% of cases but in 5% you may need transvaginal sonography (results are similar).

A video-loop function will make your life much easier.

Calipers should be able to provide measurements to 0.1mm.

2. Time:

The average time allocated per scan should be at least 10 minutes.

3. Guidelines:

If you choose to make use of the risk calculations developed by the Fetal Medicine Foundation, UK, NT-measurements must comply with the FMF protocol. FMF risk-calculations are meaningless when NT is measured in a different fashion. The FMF protocol requires:

• Proper saggital view of the fetus for CRL as well as NT-measurement.

• Fetus in neutral position (not flexed or extended since this affects measurements by up to 0.6mm)

• The minimum fetal CRL should be 45mm, maximum 84mm (gestation 11 to 13+6 weeks)

• The image of the fetus should be at least 50mm on the hard copy, better still if the fetus is larger than the picture (i.e. only the head and chest fit on the screen).

• The smallest change in caliper position should not create a measurement difference of more than 0.1mm.

• Care must be taken to distinguish between fetal skin and amnion since both structures appear as thin membranes (wait for spontaneous fetal movement away from the amniotic membrane or make the fetus jump by asking the mother to cough).

• The widest visible NT-area between the skin and the soft tissue overlying the cervical spine is chosen.

• Calipers are placed in an on-to-on fashion, i.e. they measure the full translucency: the crossbar of the caliper should be such that it is hardly visible as it merges with the white line on the border. More than one measurement must be taken and the maximum one recorded.

4. Training.

NT risk calculations change drastically with small changes in NT-measurement. NT risk calculations by the FMF are only meaningful if your measurements are EXACTLY the same as the ones by the FMF. Therefore your technique may need to be fine-tuned. The FMF offers you an external assessment and feedback on your NT-measurements . This system has been shown to very effectively increase detection rates for T21 from 30 to 75%.

Please feel free to browse through the following images which illustrate correct as well as inaccurate NT-assessments (in red).