Pregnant women who wish to exclude chromosome abnormality should
consider having amniocentesis or CVS. Ultrasound can never exclude this
diagnosis. What we can do however, using software developed by Professor
Nicolaides in London, is tell pregnant women the risk that their fetus has
a chromosome abnormality. This is calculated from their age, the thickness
of the "nuchal translucency", and the number of weeks pregnant they are
(because the thickness varies with the number of weeks).
The benefits of a scan at this stage include:
- Highest detection rate of any "no risk" test for chromosome
abnormality (including Down syndrome)
- Accurate dating of the pregnancy
- Diagnosis of multiple pregnancy
- Diagnosis of early pregnancy failure (approximately 1 in 50 women
are found on ultrasound to have failed pregnancy)
- Detection of many physical abnormalities is possible with ultrasound
at this stage.
When should the scan be carried out?
The scan is performed between 11-13 weeks. We recommend scheduling the
test when you are believed to be approximately 12 weeks pregnant from the
last period. Although the test is equally accurate for chromosome
abnormality at any time between 11 and 13 weeks, we also check the
structure of the fetus and better information on the structure can be
obtained by 12 weeks. In addition, if the booking is made for 11 weeks and
the pregnancy is found to be less advanced then rescheduling is
Will the scan be carried out through the abdomen or through the
For 9 out of 10 patients the measurement of the nuchal translucency can
be carried out with a scan through the abdomen, so this will be the first
part of the examination. If the measurement could be carried out through
the vagina then you would be offered this method instead (of course you
are always free to refuse any examination you do not wish to have). Having
finished the scan through the abdomen most women will be offered an
additional scan through the vagina since this usually gives better images
of the fetus, and therefore offers a better chance of detecting any
physical abnormality. Physical abnormalities are usually best detected at
18-20 weeks, however if you wish to have a vaginal scan many abnormalities
can be detected at this early stage.
Who will carry out the scan
The examination will usually be conducted by a sonographer (an
ultrasound technologist). The results are assessed using software
developed in London by Professor Nicolaides. Staff from Melbourne
Ultrasound for Women attended his course to ensure we used his exact
method. We have continuous ongoing audit to ensure that all operators at
Melbourne Ultrasound for Women continue to obtain reproducible
measurements. At the end of the examination you will be told whether we
consider the result to be positive or negative and the results will be
sent to your doctor.
Can I discuss the results in detail with a staff member?
If your doctor has referred you to this practice for counselling as
well as an ultrasound examination, you may see one of our counsellors to
discuss not only the results but also options for further testing.
We request your help
To enable us to monitor our results we need to know the outcome of your
pregnancy. We will give you a form to fill out at the end of your
pregnancy with a stamped addressed envelope and would be most grateful if
you would please send it back to us. With your help we can continue to
monitor and improve information for women in the future.