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Logo Ultrasound
The 11-13 Week Scan
 
Nuchal Oedema Why have a scan at 11-13 weeks?

It has been shown that a high proportion of fetuses with a chromosome abnormality such as Down syndrome can be suspected at an ultrasound examination at 11-13 weeks. All fetuses at 11-13 weeks retain small amounts of fluid under the skin around the head and neck, but there is usually a slight increase in fluid when there is a chromosome abnormality present. This is not an "abnormality" because the fluid is absorbed during the pregnancy - in fact when scans are carried out at 18-20 weeks most fetuses with a chromosome abnormality have no extra head and neck fluid. The extra fluid that we see early on is absorbed by this time. Additional fluid therefore does not harm but can be used as a marker for Down syndrome. Fluid under the skin looks black on ultrasound, and we measure the "nuchal translucency " or black area, when this area is thickened it is called "nuchal oedema" (or swelling behind the neck).

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 required.

Will the scan be carried out through the abdomen or through the vagina?

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.

Melbourne Ultrasound for Women
Level 1, 62 Lygon Street, Carlton VIC 3053
Phone: (03) 9663 3999     Fax: (03) 9663 3555
From outside Melbourne Ph: 1 800 638 810
www.melbuswomen.com.au


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North East Valley Division General Practice, Victoria, Australia, Disclaimer 
Level 1, Pathology Building, Repatriation Campus, A&RMC, Heidelberg West VIC 3081. .. map
Phone: 03 9496 4333, Fax: 03 9496 4349,  Email: nevdgp@nevdgp.org.au
Please note: NEVDGP does not provide an on-line consultation

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