Nuchal Translucency Scan

What is a Nuchal Translucency Scan?

nuchal5BIGA Nuchal Translucency Scan (NTS) is  a screening test to check for Trisomy 21 (Down syndrome), Trisomy 18 (Edwards’ syndrome) and Trisomy 13 (Patau’s syndrome).

It is an ultrasound scan done between 11.5 and 13.5 weeks of pregnancy where there is measurement of the amount of fluid in the skin at the back of the baby’s neck. It is usually performed through the abdominal wall but sometimes it is necessary to do an internal (vaginal) scan. The ultrasound has no known harmful effects on the mother or the baby.

All babies at this stage in pregnancy have some fluid in this area, but a baby with Trisomy 21 (Down syndrome), Trisomy 18 (Edwards’ syndrome) or Trisomy 13 (Patau’s syndrome) have more fluid.

This ultrasound also checks for the presence of your baby’s heartbeat, single or twin pregnancy, dates, exclusion of some other abnormalities such as exomphalos, holoprosencephaly, some heart abnormalities and megacystis.

Is a Nuchal Translucency Scan accurate?

Yes about 70%.

The accuracy is reported to increase to about 92% if combined with a blood test called the ‘maternal serum screening test’.

The maternal serum screening test measures the amount of two different proteins called PAPP-A and beta HCG that occur naturally in the mother’s blood during pregnancy. A change in the level of these proteins may indicate that there is an increased risk that the baby has Down syndrome.

What does a ‘high risk’ result mean?

One in 20 women will have a ‘high risk’ result.amnio

This does not necessarily mean that there is a problem in your unborn baby. It means that further tests should be considered to see if there is a problem with your baby. The most accurate further testing is a Chorionic Villus Sampling (CVS) or an amniocentesis. Many women instead have a Non-Invasive Prenatal Test (NIPT) as an NIPT test does not have the slight miscarriage risk of an amniocentesis or CVS. An amniocentesis or CVS test has between a one in 200 to one in 300 risk of causing a miscarriage.

What determines the cut off for ‘high risk’?

How ‘high risk’ is usually defined when the risk of miscarriage with a CVS or amniocentesis is less than the risk of the chromosome abnormality.

What does a ‘low risk’ of Down syndrome result mean?

Out of 20 women tested, 19 will have a ‘low risk’ result. This means that, with this result, the risk of having a baby with Down syndrome is very low but it does not mean that there is no risk.

It is very important to realise that this test is mostly for Down syndrome and that a ‘low risk’ result does not ensure that the baby is free of other possible birth defects.

Does the test pick up any other birth defects?

Yes. Some cardiac defects, skeletal defects and other chromosomal abnormalities can also be suspected.

Do I have to have both the NTS and maternal serum screening test?

This is recommended to improve accuracy, but the decision is your choice entirely.

Do all pregnant women have an NTS?

No. See Down syndrome screeningThe decision whether or not to have the NTS, NTS with serum, the Non-Invasive Prenatal Test (NIPT) or any testing for Down syndrome is entirely up to you.

The NIPT is becoming very popular because of its greater accuracy in excluding chromosomal abnormalities, for which it is highly unlikely to give a false positive result (compared to an NTS). It can be done earlier in the pregnancy than the NTS and you can find out the sex of your baby. While the NIPT is slightly more expensive than an NTS if the NTS is abnormal (one in 20) and there is further checking the final cost is far greater than if there had been an NIPT first off.

Where do I get the NTS done?

I recommend your NTS be done by an ultrasound unit run by obstetricians who subspecialise in pregnancy ultrasound and early foetal assessment. They are more accurate. I recommend it be done by Sydney Ultrasound for Women (Norwest Private bookings) or San Ultrasound for Women (San bookings).

I will give you a pathology referral to Douglass Hanly Moir for the serum hormone checking. If you have this done a few days prior to your NTS, the ultrasound unit will have the serum result and so can give you the full result on the day you get the NTS done.

See also

Posted by Dr Gary Sykes on -

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