One of the most remarkable things we have learnt about human reproduction is just how inefficient it seems to be!
The vast majority (probably 50-90%) of all human embryos are now known to have an abnormal number of chromosomes (the strips of DNA that contain all our genes). Embryos with abnormal chromosome numbers (too many or too few) are called aneuploid and usually don’t implant at all, or lead to miscarriages if they do. In a few rare circumstances, abnormalities such as Down’s syndrome (extra chromosome number 21) can lead to a viable pregnancy. Screening for these abnormalities has traditionally been left to nature, and in IVF to the process of culturing embryos for up to 5 days (and then selecting those that look the best under microscopy). Now though, it is also possible to biopsy embryos created by IVF and test the biopsied cell for its full chromosome number, so that information is then used to decide which embryo to put back. This is called PGD (preimplantation genetic diagnosis). The technology currently used to test the cell is called comparative genomic hybridisation (CGH), or array CGH.
This type of PGD is different to the rarer type of PGD for a specific genetic abnormality (such as screening for the genes for cystic fibrosis or Huntington’s chorea or the BRCA1 breast cancer gene). But such screening uses the same IVF technology. In the near future it will be possible to undertake both types of screening from a single biopsy.
Dr Sacks is a clinical director of IVFAustralia.