Chromosomes are tiny structures in the cells of the human body that contain roughly 30,000 to 35,000 gene pairs. The genes determine traits like eye and hair color, and also how our bodies grow and develop in the womb. Each person normally has 23 pairs of chromosomes (43 in all), inheriting one chromosome per pair from each parent. Unlike other cells in the body, sperm and egg cells only have 23 unpaired chromosomes; when a sperm and egg unite, they form a fertilized egg with all 46 chromosomes. But sometimes, in the process of cell division, an error occurs, so that a sperm or egg cell has too many or too few chromosomes. Scientists don't know what exactly causes this, but the resulting embryo has a chromosomal abnormality. About 70 percent of such pregnancies result in a miscarriage, but if carried to term, the baby could have any number of disorders because of the abnormality.
Down syndrome is the most common of these disorders, affecting about 1 in every 800 to 1,000 live-born babies. Down syndrome babies have three copies of a particular chromosome instead of two. Children with Down syndrome suffer from mental retardation, heart defects, and overly round, smooth facial features.
Babies born with extra copies of other chromosomes can be severely retarded with many physical birth defects; most die before their first birthday. Other chromosomal abnormalities include small missing sections of the chromosome; a missing single gene; a section of one chromosome attaching itself to another; and a chromosome that is somehow snipped out and reinserted upside down in the sequence.
What is hCG ?
Human chorionic gonadotropin (hCG) is a hormone produced by a woman's placenta during pregnancy, released into the blood and urine. It helps maintain the pregnancy and helps the development of the fetus. HCG levels are what is measured in pregnancy tests. When measured along with other hormone levels, hCG levels can also be used to detect Down syndrome.