Non invasive prenatal testing
The Harmony test is a blood test that looks for whether your baby has Down’s Syndrome, Edward’s or Patau’s Syndrome. In these conditions there are 3 copies of certain chromosomes instead of 2 (“Trisomy”). The test looks at DNA from the baby that is present in the mothers bloodstream.
You can have a Harmony after your dating scan.
Test performance for Down’s Syndrome
This screening test has a high “sensitivity” for Down’s syndrome (Trisomy 21) of over 99%. This means that the harmony test will pick up more than 99% of babies who have Down’s syndrome.
Because the sensitivity is not 100% there is a small chance (less than 1%) that a harmony test may report a low probability of the baby being affected when in fact the baby does have the condition.
Another aspect of test performance is its “positive predictive value” (PPV). This is the likelihood that a result stating a high chance that the baby has the condition, is correct.
The PPV depends upon the background risk. If you have had a high chance result for Down’s syndrome from an NHS screening test or are in an older age group the positive predictive value will be above 90%. If you are low risk the positive predictive value will be much lower, again the risk depends on factors such as your age. One large study reported the PPV for low risk women to be 82% for Down’s syndrome.
Therefore for high chance harmony test results an invasive test such as amniocentesis or chorionic villus sampling is recommended to give diagnostic results.
Test performance for Edward’s and Patau’s Syndromes
Edward’s syndrome (trisomy 18) and Patau’s syndrome (trisomy 13) are rarer conditions. This means it is less possible to be certain of test performance. The reported sensitivity for these is 97% (T18) and 94% (T13), meaning that for Edward’s syndrome, approximately 3% of babies who have the condition will not be detected on Harmony testing and for Patau’s 6% of babies who have the condition will not be detected with the Harmony test. The PPV of the test is 37% for Edward’s syndrome and 49% for Patau’s syndrome.
Reasons for choosing Harmony
The Harmony test is available early in the pregnancy, from 10 weeks onwards.
Many couples now choose to pay for a non-invasive prenatal test (NIPT) such as Harmony because of its high sensitivity for Down’s syndrome.
Another reason for choosing an NIPT is after receiving a high chance result for Down’s syndrome from the 12 week NHS screening test. In many cases the baby will not have Down’s syndrome. NIPT is another option as an additional step prior to invasive testing. Invasive tests are reported to have a 1 in 200 risk of miscarriage. Many who opt for an NIPT as the next step will be reassured by a low chance result. Some couples will receive a high chance result and then proceed to an invasive test.
Non-invasive prenatal testing does not diagnose other genetic or structural diseases such as spina bifida. A low risk result obviously does not guarantee a healthy baby.
In 3% of cases the sample cannot be processed due to it having insufficient DNA within it. In this case we will contact you to arrange repeat testing at no additional charge.
Twins & IVF pregnancies
The Harmony test can be used for IVF pregnancies, including those with donor eggs.
It can be used in twin pregnancies for screening purposes but not to look at the sex chromosomes. It cannot be used in twin pregnancies where one twin has not been viable. It is also not validated for use in triplet pregnancies.