The genetic ‘code’ that makes us human is carried in each and every of our cells on deoxyribonucleic acid (DNA) molecules which form our chromosomes. To a great extent our DNA determines our health and development through life and humans cells should contain 46 chromosomes on 23 pairs, with each half of the pair coming from each parent. The only exceptions to this is are the sperm and egg cells which of course have half the normal amount of DNA on 23 (unpaired) chromosomes in them, with a baby getting half it’s DNA from each parent. Both egg and sperm cells are made in a special process called meiosis, where the DNA in a normal cell is copied and it then divides twice (rather than once) which results in four cells; each of them with half the normal number of chromosomes and DNA.
However, it’s not easy to do perfectly and even 2-13% of sperm from normally fertile men don’t carry the right amount of DNA on the right number of chromosomes, as they either have extra or missing chromosomes (or pieces of chromosomes), and the medical term for this is “aneuploidy”. Although many of these sperm will be able to fertilise an egg, the abnormalities in their genetic coding means the baby won’t be as healthy as normal. Both eggs and sperm can have “aneuploidy” on any of the 23 pairs of chromosomes, but for most of the chromosomes the consequences are so significant it inevitably leads to a failed pregnancy. However there are some chromosomes that can support an aneuploid pregnancy that goes to term, and this happens naturally in 1 of 160 live births.
The usual way to see if sperm are carrying the correct amount of genetic material is with a ‘fluorescent in situ hybridisation’ (FISH) test that looks directly at the sperm’s chromosomes. The amount of aneuploidy is often higher in infertile men, and certain abnormal sperm types have an increased rate of chromosomal abnormalities.i This is particularly true of ‘oligozoospermia’ (abnormally low sperm numbers) as about half the men with this have much higher rates of chromosomal abnormalities. Aneuploidy rates vary greatly in the population and change over time for individuals, and there are some men who have many aneuploid sperm in otherwise perfectly normal semen samples.
We know that aneuploidy (in both eggs and sperm) is the major cause of miscarriage (over 50%) and that miscarriage rates increase with the age of the parents, which matches the increased chance of having children with chromosomal abnormalities for older parents, so it is a considerable (and chance) issue. Testing is usually for just 5 of the 23 chromosomes: 13, 18 and 21 and the two sex chromosomes (X and Y) as it’s only these chromosomes that can accept aneuploidy and result in a live birth. These babies will have chromosomal problems such as Down’s syndrome (chromosome 21), Edwards syndrome (chromosome 18) or conditions that relate to their sexual development.
If the FISH test does reveal high levels of genetically-abnormal sperm, it’s usually possible for