FAQs - Section 8: Glossary

A procedure in which the obstetrician inserts a fine needle into the amniotic fluid and collects a small sample for analysis.

The presence of an abnormal number of chromosomes in a cell or fetus. There are normally two copies of chromosomes 1 to 22. The sex chromosomes (X and Y) are different in that a woman normally has two X chromosomes, and a man has one X and one Y.

A procedure in which the obstetrician inserts a fine needle into the placenta and collects a small sample of tissue for analysis.

The genetic code in almost every living thing is in the form of DNA. DNA is a very long chemical that would look like a twisted ladder if we had a sufficiently powerful microscope. The DNA in a single human cell is very thin (one billionth of a metre across) and very long; there are two metres of DNA in every human cell. When a cell divides into two cells, the DNA must be copied (making it 4 metres long) and then separated into two equal sets of genetic instructions (2 metres each) before the cell can split into two. The cell manages this extraordinary feat by packaging the long DNA into a number of bundles, 46 in all. These bundles of DNA are large enough to be seen in the microscope and are called chromosomes. The packaging of DNA into chromosomes in a person’s cells is always done exactly the same way. This means that a particular chromosome in different cells always has the same portion of the genetic code. An abnormality in the number of that particular chromosome in every cell results in the genetic code in every cell being abnormal in the same way.

This term typically refers to the combination of analysis of a sample of maternal serum and an ultrasound of the fetus. The two investigations are an established method of identifying women who are at increased risk of having a baby with physical or genetic abnormalities.