Read on if considering this test…..
Non-Invasive Prenatal Tests (NIPT) are blood tests for pregnant women which give a near-diagnostic level of accuracy for the commonest chromosome problems in pregnancy – trisomies 21, 18 and 13, as well as the option to check for sex chromosome anomalies. Results are based on ratios of the “fetal” fraction of cell-free DNA in the mother’s blood.
NIPT can be performed from 10 weeks for women who have decided to proceed regardless of the results of first trimester screening. The earlier in pregnancy the test is performed, the greater the chance of not getting a result, in which case the test is usually repeated at no further cost.
The test is suitable for moderately high risk situations. However where a definite problem or very high risk scenario is identified, NIPT is not usually the ideal choice, because it is not a definitive test and it does not cover the full range of chromosome problems – 20% of significant chromosome abnormalities are not checked.
The test is already being used for low risk women as it is the most accurate test available for trisomies that does not carry any direct miscarriage risk. We or the referring doctor can provide guidance, sometimes in conjunction with the 12 week ultrasound for structural abnormalities.
Decision about how much you wish to test
NIPT is mainly to check for trisomies. You will need to decide whether to also have the sex chromosomes checked for anomalies – a separate decision from whether you want to know the gender. This group covers a variety of conditions ranging from very mild to severe problems. Some can be better managed in childhood if the chromosome anomaly is identified from birth rather than first diagnosed later in life.
Microdeletion screens offered at extra cost by some labs can have more problems with accuracy, both false positives and negatives.
It is important to understand that if a chromosome problem such as Down’s is detected on NIPT, a definitive test (amnio or CVS) is then required, since there are occasional false positive results.
Limitations of NIPT:
The test only covers the 80% most common chromosome defects found in pregnancy including trisomy 21, trisomy 18, trisomy 13 and potentially, abnormalities of the sex chromosomes. It is not designed to pick up mosaicism (where only some cells are affected) or partial chromosome deletions. Although NIPT is called a “fetal” test, it is really mainly placental DNA that is analysed. This is not usually a problem with the common trisomies but requires careful consideration if a sex chromosome anomaly is found.
Many fetal health problems are not covered by NIPT, so the usual 12 week and 19 week ultrasounds are still recommended for structural information.
The main differences in choice of lab are
At Park Ultrasound we have not aligned with a single lab, so the choice of lab can be tailored for each clinical situation.
Twins – NIPT can be used, even though it does not work as well as in single pregnancy.
These tests are very rapidly developing, please discuss any specific queries.