Chromosomal abnormalities are irregularities visible under light microscopy in the number or structure of chromosomes, 23 pairs being the number normally present in humans. These abnormalities usually occur by chance during maturation of the egg or sperm cell; less often they are hereditary irregularities. Most de novo chromosomal abnormalities are so severe that they lead to miscarriage. The most frequently encountered chromosomal abnormality which the fetus survives is trisomy 21 (Down syndrome). Other chromosomal abnormalities occur less often, and the conditions they cause are sometimes more severe than trisomy 21 (e.g., trisomy 13, trisomy 18), and sometimes less severe (e.g., changes in sex chromosomes).
A handicap in the child due to the faulty arrangement of chromosomes can occur in pregnant women of any age. However, the probability increases with maternal age. The probability of giving birth to a child with trisomy 21 (Down syndrome) is 1:1600 for mothers aged 20 years, 1:900 for those aged 30 years, 1:350 for 35-year-olds, and 1:100 for mothers aged 40 years.
It is only possible to confirm the presence or absence of a chromosomal abnormality with absolute certainty by means of chromosomal analysis! This can be done, for instance, using cells taken from the amniotic fluid (amniocentesis) or from placental tissue (chorionic villus sampling).
The current recommendation is that pregnant women who are 35 years or over when they give birth should consider undergoing amniocentesis or chorionic villus sampling. Because of the inherent risk of these invasive procedures and due to the fact that about 50% of pregnancies with chromosomal abnormalities occur in women under 35 years, in many cases parents want an estimation of their individual risk, and this is possible using various techniques such as ultrasound and/or blood tests.