Karyotype tests (or chromosome analysis) check the structure, number, arrangement of chromosomes and the DNA code it carries and is essentially the same test that’s performed when ‘amniocentesis’ or ‘chorionic villus sampling’ (CVS) is used during a pregnancy to check a baby’s health or sex, but it uses a blood sample. The chromosomes are separated, processed and photographed to see if there are any missing or extra chromosomes, for tructural changes that cause pregnancy failures, or genes that cause illness.
Karyotype testing can also reveal defective chromosomes carried by parents that could cause problems for their baby’s development or growth and even if both partners are healthy one or other of them might be carrying genes that increases the risk of serious defects because some genes are “recessive” (this means that it’s carried but not ‘expressed’ in a person) but when both parents happen to carry the recessive gene there’s a high chance it will affect the health of their offspring.
A high percentage (around 50%) of pregnancy losses in the first trimester are due to genetic problems and for couples who experience repeated miscarriages without a known cause, or who have great difficulty getting pregnant at all, it may be that genetics play a part and the only way to know is by testing. When this is done it’s important that both partners are tested so that a full picture can be obtained as there are some rare genetic matches that significantly reduce the likelihood of pregnancy. This includes HLA DQα genotype compatibility where there are no genetic problems as such, but an unfortunate and crucial co-incidence of matching genes.
The number of abnormal numbers of chromosomes (trisomy which cause genetic conditions such as Down’s) in egg and sperm cells increase as people get older, and this plays a large part in the higher miscarriage and lower pregnancy rates that are experienced by older couples. However this is a natural variation that comes with age and each egg or sperm cell will be affected differently and is a separate issue to what the karyotype test will be looking for. The karyotype test is also unable to detect DNA fragmentation, as again this is something that varies between a person’s cells.
It’s really important for couples to discuss the pros and cons of having the tests (preferably with a specialist counsellor) before having them, as the results can be challenging and emotionally difficult.