Chromosomal abnormalities

Chromosomes are tiny structures inside the centre of all cells that come in pairs, one from each parent. Each chromosome carries many genes and it is these genes that define all of a person's physical characteristics (such as sex, blood type, hair & eye colour... to mention just a very few). Having too much or too little of a chromosome causes an imbalance in gene activity.

One-off pregnancy losses can occur when the cells of the placenta and the fetus contain an abnormal number of chromosomes. Mostly the extra chromosome or the deficient chromosome is not present in the parents: instead the abnormality occurs as mature eggs or (less often) sperm cells are formed just before they are released

The miscarriage happens by chance. In most cases, there is nothing wrong with the mother or father's health and miscarriage is not likely to occur again in a later pregnancy.

There is, however, an important exception - balanced translocations

Balanced translocations

A person with a balanced translocation has the correct amount of each chromosome and so has no outward signs of genetic abnormality. However, their chromosomes are arranged incorrectly, which will cause problems when the chromosomes divide - as in the creation of sperm and eggs.

The diagram below shows the possible outcomes when the chromosomes of a person with a translocation divide.

If a translocation is suspected as the cause for recurrent pregnancy loss, both parents can be tested and, if possible, products of conception can be tested as well to confirm the finding.

Upon confirmation, there are several options. As can be seen from the diagram above, there is a 50% chance the embryo will have a normal balance of chromosomes - although there is a chance that it will carry a translocation itself. If the couple continue trying to get pregnant naturally, these odds determine their chance (other factors aside) of carrying a child to term and that child being healthy.

An alternative is preimplantation genetic diagnosis (PGD) . In this procedure, the couple undergo a course of IVF and the resulting embryos are tested as they develop in the laboratory. In this way, embryos with unbalanced chromosomes can be avoided.