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Prenatal Diagnosis

Less Invasive Methods

An ultrasound scan bounces soundwaves off of the fetus to create an image. A scan is often performed after the sixteenth week of pregnancy, and the anatomy and size of the fetus is measured to see if it is growing and developing normally. The scan can often detect major structural problems, such as a malformed heart or spine. An unusual finding on an ultrasound scan can be a warning to investigate further. However, not all birth defects can be detected by ultrasound.

An ultrasound is sometimes done at weeks five or six to confirm that a pregnancy is present. This early, the embryo looks like a lima bean with a pulsating blip in the middle, which is the beating heart. Ultrasound performed late in pregnancy can provide clues to the approaching birth date. New three-dimensional ultrasound scans offer spectacular views of the fetus.

Another noninvasive method to detect fetuses at risk for some birth defects is maternal serum marker screening. A sample of blood from a pregnant woman taken at approximately weeks 15 to 18 is analyzed for the amount of several substances, including alpha fetoprotein (AFP); a form of estrogen called unconjugated estriol; and human chorionic gonadotropin (hCG), a hormone produced only during pregnancy.

Maternal serum screening began in the 1970s with the AFP test, invented by a man whose son was born with a neural tube defect. High levels of AFP in a woman's blood indicate an increased risk for a neural tube defect in the fetus. The neural tube forms by approximately day 28 of gestation, when a portion of the flat embryo (the neural plate) folds to form a tube that will develop into the brain and spinal cord. The tube normally closes up like a zipper starting at several points along its length. If a hole remains, the brain and spinal cord underneath are exposed, causing damage.

Several years after the AFP test was developed for neural tube defects, researchers noted that low AFP correlates to an increased risk that a fetus will have an extra chromosome, particularly at positions 18 or 21. This condition is called a trisomy. Trisomy 21, an extra chromosome 21, is the most common cause of Down syndrome. Over the years, analysis of other substances have been added to refine this test, which is now offered routinely to pregnant women. Abnormal results on maternal serum screening tests indicate that amniocentesis should be done to diagnose a neural tube or chromosome defect, and that genetic counseling should be offered.

Additional topics

Medicine EncyclopediaGenetics in Medicine - Part 3Prenatal Diagnosis - Viewing Chromosomes, Less Invasive Methods, Preimplantation Genetic Diagnosis, Genetic Counseling And The Ethics Of Prenatal Diagnosis