NCMG co-ordinates pre-natal testing for families known to have a genetic disorder. The fetal assessment unit does not request the assistance of NCMG when prenatal testing is carried out because of advanced maternal age. The Genetic counsellors co-ordinate approximately 100 prenatal tests for families annually. This number is increasing year on year. Most families are known to us and self refer to access antenatal testing.
Others are referred in pregnancy and we can establish risks in time for a pre-natal test in some cases. The majority of couples opting for testing face a high risk (25% or greater) of recurrence.
The Genetic counsellor liaises with the family before testing to ensure a complete understanding of what the testing process entails and how the couple wish to learn of the result.
The Genetic counsellor requests an appointment with one of the three Dublin fetal assessment units based on the date of the patients last menstrual period (LMP).
As many samples need to go abroad and timing is of utmost importance, all pre-natal tests organised by NCMG are performed in Dublin. We aim to get a result back to the couple as soon as possible to allow an option regarding decisions on rest of pregnancy care and management.
However, up to 25% of requests for pre-natal testing to NCMG are for disorders where there is a low recurrence risk (<1% to ~<5%). These requests tend to come from families who have had a child with a severe genetic disorder and the couple cannot contemplate a recurrence.
Most of the pre-natal tests we request are done by chorionic villus sampling (CVS) as this gives a sample that produces good quality DNA that can be extracted speedily. The optimal time to perform this test is between 11-13 weeks.
Antenatal tests carried out before week 11 risk higher rates of maternal cell contamination (MCC) and failure due to sample dissection difficulties. NCMG cytogenetic and molecular laboratories process the CVS samples. A result on an at-risk fetus can be reported back to the couple within 2-3 weeks in most cases.
The cytogenetic laboratory analyses the chromosomal complement of the fetus (termed karyotyping). The main antenatal referral requests to the cytogenetics laboratory come directly from obstetricians because of concerns on antenatal scanning or the sample has been taken for maternal age/anxiety.
The molecular laboratory processes samples for single gene genetic testing (cystic fibrosis, spinal muscular atrophy, sickle cell anaemia). Whilst the latter three disorders can be analysed locally (they are the most common requests for pre-natal testing we handle), we also get requests for testing of other less common genetic disorders.
As we cannot test for all genetic conditions in-house, testing for these conditions takes place in up to 200 different foreign diagnostic laboratories. Samples are couriered with appropriate documentation to avoid delays in customs.
The molecular laboratory also performs Maternal cell contamination testing on all of our pre-natal tests to ensure that the sample that has been tested is derived from fetal DNA, and not from the mother which would give an erroneous result.
The clinical team at NCMG do not get involved with these referrals. NCMG clinical staff co-ordinate testing for between 30-40 families who have had a previous child with a rare chromosomal anomaly (e.g. unbalanced chromosomal translocation). These require liaison with the cytogenetics laboratory as often special Fluorescent In Situ Hybridisation (FISH) probes are required for the genetic analysis. These are high risk cases and the analysis is targeted at the familial abnormality- it is not a routine chromosome analysis.
There are some chromosomal disorders where an amniocentesis is preferred to a CVS because of the quality of the chromosome test. We advise the fetal assessment unit accordingly.