Amniocentesis Health Article

Results

It is important to emphasize that normal results from tests performed on amniotic fluid do not necessarily guarantee the birth of a normal infant. Each couple in the general population faces a risk of roughly 3-4% of having a child with any type of congenital birth defect. Many of these will not be detected with tests performed on amniotic fluid samples obtained by amniocentesis. Babies with birth defects are often born into families with no history of genetic disorders.

Chromosome karyotyping is interpreted according to standardized nomenclature, International System of Cytogenetic Nomenclature (ISCN). The system describes the number of chromosomes, the sex chromosomes, and then any abnormalities seen in specific chromosomes using a numerical system to identify banding characteristics. Bands are numbered starting at the centromere. Standardized symbols and abbreviations describe the type of abnormality. For example, a + sign designates an extra chromosome and 47,XY,+21 designates a male with an extra chromosome 21. The designation 46,XX, t(3:5)(q21;p15.2) means that the subject is a female with a normal number of chromosomes. There is a translocation (exchange) between chromosomes three and five involving band 21 of the long arm of number three and band 15.2 of the short arm of chromosome five.

Amniotic fluid is normally clear or slightly turbid. The concentration of AFP is reported as multiples of the median (MOM). Normal values are dependent upon the sampling time. For example, the median AFP at 15 weeks is 16.3 micrograms per mL and at 20 weeks is 8.1 micrograms per mL. Bilirubin is also time-dependent normally decreasing during gestation. The concentration should be below 0.075 mg/dL at 20 weeks and below 0.025 mg/dL at 40 weeks (full term).

An L:S ratio of 2.0 or higher correlates with fetal lung maturity. Lecithin greater than 0.10 mg/dL also correlates with fetal lung maturity.

Health care team roles

Actual samples of amniotic fluid or placenta are obtained by persons with specialized training. These are usually obstetricians or gynecologists with additional training. Sonographers assist in amniocentesis. Specially trained nurses prepare women for amniocentesis. Cytogenetic technologists, CLSp(CG) perform cell culture, karyotyping, and DNA analysis of fetal cells. Clinical laboratory scientists/medical technologists perform biochemical testing, and may also perform DNA and some molecular tests on fetal cells from the fluid. Pathologists with special training in genetics interpret results. Obstetricians or geneticists provide results to a woman or couple and discuss treatment options that may be available. Genetic counselors and other mental health professionals may provide additional support and counseling to a woman or couple after the results of amniocentesis have been returned.

KEY TERMS

Alpha fetoprotein—A protein found in amniotic fluid that is commonly used to screen for certain structural birth defects.

Amnion—A thin, transparent membrane that holds the fetus in the amniotic fluid. The amniotic sac is sometimes called the bag of waters.

Anesthetic—Drug used to temporarily cause loss of sensation in an area of the body. An anesthetic may either be general, associated with a loss of consciousness, or local, affecting one area only without loss of consciousness. Anesthetics are administered via inhalation, topical application or needle injection.

Aneuploid—A fetus with an abnormal number of chromosomes.

Chorion—The outer membrane of the amniotic sac. Chorionic villi develop from its outer surface early in pregnancy. The villi establish a physical connection with the wall of the uterus. The chorionic villi eventually develop into the placenta.

Chromosome—A linear thread of genetic material contained within every cell. Humans have 46 chromosomes arranged into 23 distinct pairs. Each parent contributes one set of chromosomes, or 23, to a child. Changes in the total number of chromosomes, the shape and size (structure) of a chromosome or the contents of a chromosome may lead to abnormalities in the amount of genetic material. These abnormalities often lead to abnormal physical and mental development.

Conceptus—The product of conception, or the union of a sperm and egg cell at fertilization.

Cystic fibrosis—An inherited disease characterized by repeated lung infections, functional abnormalities of the pancreas, and an elevated level of salt in sweat. Individuals with cystic fibrosis require long- term aggressive medical care. Survival into adulthood is common, in part due to advances in treatment. Death, however, is frequently caused by respiratory failure. Although cystic fibrosis is more common among Caucasians, it has been reported in individuals of other races.

Down syndrome—A genetic condition characterized by moderate to severe mental retardation, a characteristic facial appearance, and, in some individuals, abnormalities of some internal organs. Down syndrome is always caused by an extra copy of chromosome 21, thus there are three rather than the normal two. For this reason, Down syndrome is also known as trisomy 21.

Fetus—The term used to describe a developing human infant from approximately the third month of pregnancy until delivery. The term embryo is used prior to the third month.

Fibroid—A non-cancerous tumor of connective tissue. It is made up of elongated, threadlike structures, or fibers, which usually grow slowly and are contained within an irregular shape. Fibroids are firm in consistency but may become painful if they start to break down or apply pressure to areas within the body. They frequently occur in the uterus and are generally left alone unless growing rapidly or causing other problems. Surgery is needed to remove fibroids.

Sickle cell anemia—An hereditary form of anemia due to abnormal sickle-shaped red blood cells. As a result, the cells cannot efficiently carry oxygen to body tissues. Common clinical features of sickle cell anemia include severe pain in the joints and abdomen, swelling of the tops of hands and feet, and fever. Sickle cell anemia is frequently found among individuals with ancestors who lived in central Africa.

Tay-Sachs disease—An inherited biochemical disease caused by lack of a specific enzyme in the body. In classical Tay-Sachs disease, previously normal children become blind and mentally handicapped, develop seizures, and rapidly decline. Death often occurs between the ages of three to five years. Tay-Sachs disease is common among individuals of eastern European Jewish background but has been reported in other ethnic groups.

Trimester—A three-month period. Human pregnancies are normally divided into three trimesters: first (conception to week 12), second (week 13 to week 24), and third (week 25 until delivery).

Uterus—A muscular, hollow organ of the female reproductive tract. The uterus contains and nourishes an embryo or fetus from the time the fertilized egg is implanted until birth.