Lecture Notes
Antenatal screening

Are you requesting NT scans and/or Harmony tests in some or all of your pregnant patients? Dr Simon Winder gave us a clearer approach to navigating these alternatives at the October medical update.

There is a concern from the Obstetricians that not all GPs are offering antenatal screening to all women. All pregnant women should be offered screening. Younger mothers may have lower risk, but low risk is not no risk. If a woman does choose to have screening, she may now choose the traditional combined screening test (NT + blood test) and/or NIPT (Harmony).

The combined 12 week NT ultrasound and maternal blood test is still an important test. Anencephaly, exomphalos, gastroschisis and megacystitis are just some of the conditions that can be detected at 11-13 weeks on ultrasound. These conditions are not detected by the newer NIPT options. Fetal anatomy is best visualised at 12 weeks. Nuchal Translucency (even with maternal blood testing) does have a relatively high false positive rate, which generates considerable patient anxiety and leads to invasive testing.

The NIPT (Harmony) measures cell-free DNA to screen for Trisomy 21, 18 and 13 and sex chromosome aneuploidies. This test is expensive and is not funded by Medicare. It can be done as early as 10 weeks and 1 day. Results now take around 3-5 days, which is much less than previously as it doesn’t have to be sent overseas any more. Interpreting the results comes back to the woman’s underlying risk. The high negative predictive value means that if the NIPT is normal, there is an excellent chance the baby will be normal for those abnormalities, but it does not cover all abnormalities. However, in a low risk woman, an abnormal NIPT can still occur with a normal pregnancy. For this reason, any abnormal NIPT should be followed by diagnostic testing with amnio/CVS. There is also a failure rate, i.e. insufficient DNA detected. Obese women have 3-4x higher failure rate.

Overall, all women should be offered the choice of NIPT or combined screening. If they choose NIPT, they should still have an ultrasound at 12 weeks. One option is to do the routine screening at 12 weeks, then consider NIPT if this screening finds an increased risk because this may save them from an unneccessary amnio / CVS. In future, there is a possibility that NIPT may become subsidised for women who have returned a higher risk NT result.

Patient information about the Harmony test can be found here.

As a side note, Dr Winder pointed out that there is no routine requirement for an ultrasound at 34 weeks. This is a persistent issue for the Illawarra region.

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