Prenatal diagnosis

Diagnosis before birth. Methods for prenatal diagnosis include ultrasound of the uterus, placenta, and/or developing fetus; chorionic villus sampling (CVS) to obtain tissue for chromosome or biochemical analysis; and amniocentesis to obtain amniotic fluid for the analysis of chromosomes, enzymes, or DNA. A growing number of birth defects and diseases can be diagnosed prenatally, and in some cases treated before birth. Also known as antenatal diagnosis.

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(antenatal diagnosis)

diagnostic procedures carried out on pregnant women in order to discover genetic or other abnormalities in the developing fetus. Ultrasound scanning (see ultrasonography) remains the cornerstone of prenatal diagnosis. Other procedures include estimation of the level of alpha-fetoprotein in the mother's serum and the amniotic fluid; chromosome and enzyme analysis of fetal cells obtained by amniocentesis or, at an earlier stage of pregnancy, by chorionic villus sampling (CVS); and examination of fetal blood obtained by fetoscopy or cordocentesis. If the results indicate that the child is likely or certain to be born with severe malformation or abnormality, the possibility of abortion is discussed by the doctors involved with the parents.

Until recently, prenatal genetic diagnosis was dependent on CVS and amniocentesis, but it may now be extended to the earliest stages of embryonic development, before implantation occurs (preimplantation genetic diagnosis; PGD). Access to these early embryos requires the removal and in vitro fertilization of egg cells: three days after fertilization one or two cells are aspirated from the six- to eight-cell embryo; alternatively, tissue is removed from an embryo at five or six days, when it has reached the blastocyst stage. Isolated cells can then be genetically analysed, using the polymerase chain reaction amplification technique to produce enough DNA for genetic diagnosis. Even before pregnancy, this technique can be used to enable a prospective mother to be identified as the carrier of a gene defect, by means of the mouthwash test. When a defect is detected, genetic counselling is offered.