Chromosome abnormalities can be screened for by NIPT (non-invasive testing). Only an invasive test can confirm the high risk result from a screening NIPT test.
Invasive testing includes chorionic villus sampling or amniocentesis.
The type of invasive test performed will depend on the findings at NIPT and/or ultrasound findings from a morphology scan.
There are 3 methods of testing depending on the clinical indication:
- Conventional banded karyotype – for sex chromosomes disorders, common aneuploides, unbalanced chromosome rearrangements, balanced chromosome rearrangements or suspected mosaicism.
- Microarray – this detects gain or loss of genetic material that is smaller than that which can be detected by a banded karyotype and covers all chromosomes. It cannot detect balanced rearrangements. It is indicated if there are structural abnormalities detected by ultrasound.
- FISH (fluorescence in situ hybridization) is used for microdeletion syndromes and is a rapid method of testing for common chromosome trisomies.
Miscarriage
At least 60% of first trimester miscarriages are due to a chromosome abnormality, the majority of which are not inherited. Karyotyping the miscarriage material will be able to give an answer for the pregnancy loss in 60% of cases. Follow up genetic testing of the parents will be indicated in a small number of these cases.
Miscarriage material can be tested by conventional karyotype and if this fails microarray. Conventional karyotype helps in cases where there is a balanced structural rearrangement (which array cannot detect) as well as extra or missing genetic material.