Aside from a physical condition that can cause infertility, genetics can also play a part. Genetic abnormalities can affect an embryo's ability to grow, implant and develop, causing infertility or miscarriage.
Genetic mistakes can occur during a number of stages of embryo development. In many cases, these mistakes will cause an embryo to cease development and miscarry - often before the mother even knows she is pregnant.
In other cases, such as with Down syndrome, the abnormality may not be severe enough to prevent the fetus developing and being born, although they will result in a child with disabilities.
Down syndrome (named after English physician John Langdon Down) is most commonly caused by an error occurring when the egg or sperm cell divides. At that time, extra material from chromosome 21 is given to the embryo. As the embryo develops, this extra chromosome is replicated in every cell, causing abnormal development.
This kind of genetic abnormality is called a trisomy (tri = three, somy = chromosomes) and you will often hear Down syndrome referred to as trisomy 21. Trisomies of other chromosomes do happen, but most result in fetuses too abnormal to develop.