Nuchal Translucency scan got it wrong

A 29 year old patient in her second pregnancy had a Nuchal Translucency ultrasound Nuchal Translucency Scanscan (NTS) to check whether her baby has Down syndrome (Trisomy 21). The serum test maternal serum screening (PAPP-A and beta HCG levels) was also done to increase the accuracy of the testing. The testing was done when she was 12 week and 2 days pregnant by dates. The result was 1 in 2862 risk of her baby having Down syndrome

When she was 19 weeks and 2 days pregnant she had a routine foetal morphology scan of her baby. It was noted that her baby had bilateral mild talipes and bilateral mild bilateral pyelectasis. I was contacted by Obstetric Ultrasound subspecialist who did the scan. He suggested that she has an amniocentesis to exclude her baby having Trisomy 21.

Trisomy 21 (Down syndrome)The amniocentesis was done the same visit as the morphology scan. The rapid FISH result was consistent with a male infant with Trisomy 21. This was confirmed by the full karyotype result.

The patient had her husband were devastated. To find out that their baby had Down syndrome was extremely upsetting. But to find this out half way through the pregnancy and a very reassuring NTS and serum screening result was so distressing.

It is reported that the NTS with serum has 92% accuracy in detecting Trisomy 21. This patient’s result was in the other 8% when the NTS with serum is not accurate

It is for mainly this reason and as well the relatively high number of false positive NTS and serum results (1 in 20) that more women are choosing the new Non-Invasive Prenatal Foetal and Maternal DNA DiagramTest (NIPT) instead of the NTS and serum.

While the out of pocket cost with the Harmony NIPT test is $520, the accuracy is far greater than the   NTS and serum. A negative NIPT result is consisted with a < 1 in 10,000 risk of the baby having Trisomy 21. It is highly unlikely for there to be a false positive result

As well as also checking for Trisomy 13 and 18, the NIPT checks in for sex chromosomes disorders. This is not possible with the NTS. Checking for sex chromosomes disorders means the couple can very accurately find out the sex of their baby if they wish to know.

The NIPT testing is possible earlier than the NTS. The NIPT can be done at 10 weeks pregnancy whereas the NTS is done at 12 weeks pregnancy

So with the NIPT is possible for a couple to know before 12 completed weeks of pregnancy and with a very high degree of confidence whether or not their baby has any of the commonest chromosomal disorders and also the sex of their baby!

Posted by Dr Gary Sykes on - Tests in Pregnancy

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