(b) : Amniocentesis is a foetal sex determination and disorder test based on the chromosomal pattern in the amniotic fluid surrounding the developing embryo. It is withdrawal of a sample of the amniotic fluid surrounding the embryo in the uterus by piercing the amniotic sac through the abdominal wall. Amniotic fluid drawn from the uterus contains cells from the embryo (mostly shed from the skin). Cell cultures enable chromosome patterns to be studied so that prenatal diagnosis of chromosomal abnormalities (such as Down's syndrome) can be made. Metabolic errors and other diseases such as spina bifida, can also be diagnosed prenatally from the biochemistry of the cells.