What is infertility?
Pregnancy rates and age
Why do success rates decline with age? There is no simple answer, but it seems likely that the most important reason is the quality of the egg - and therefore the quality of the embyro after fertilisation.
Studies analysing the genetic and chromosome make-up of eggs and embyros have consistently found that the rate of defects increases with age. This is most clearly illustrated in the increasing risk of Down's syndrome seen in older mothers.
Down's syndrome is a condition caused only by an abnormality in which there are three copies of chromosome 21, not two as there should be. In some rare cases the result is an inherited abnormality, in others these chromosomal defects prevent the embryo from implanting in the uterus (whether in natural or assisted) conception.
As seen in the graph below, the rate of chromosome abnormality increases as the implantation rate decreases.