Prenatal Diagnostic Techniques
Prenatal diagnosis, assessing the condition of a fetus before it is born, has become an important--although controversial--part of pregnancy care. Increasingly sophisticated technology adds to the variety and accuracy of prenatal tests.
The earliest prenatal testing was simple. The mother noted activity inside her womb; the physicians felt it from the outside. Machines were developed for listening to the fetal heartbeat and diagnose fetal "stress;" the 1950s saw development of amniocentesis and the ability to culture cells from amniotic fluid in the 1960s allowed identification of chromosomal disorders--specifically Down's syndrome--and today, a wide range of metabolic and chromosomal disorders can be detected through tests such as ultrasound, chronic villus sampling (CVS), Bart's triple test, maternal serum screen (MSS)/alpha-fetoprotein (AFP), umbilical vein sampling/cordoncentesis, fetoscopy, and genetic counselling.
Ultrasound scanning has had the most comprehensive effect on prenatal testing. Sound waves produce a picture of the developing fetus, and the procedure has no known harmful effect. The development of real-time ultrasound (1977) gave detailed images of the moving fetus, permitting invasive diagnostic techniques through visual guiding of needles into the womb for samples blood or tissue while avoiding damage to the fetus, placenta, or other vital structures.
Nuclear magnetic resonance imaging (MRI) and spectroscopy (NMR) reveal biochemical information about fetal soft-tissue and organ structure and function. Amniocentesis, normally recommended at 16-18 weeks' gestation, is sometimes conducted earlier (11-12 weeks); however, at this point fetal loss is increased by 7.6%, and the risk of related negative consequences is higher.
Chorionic villus sampling involves collecting a small sample of the developing placenta for chromosomal, biochemical, and DNA content. Obtaining chorionic villi through the cervix was described by Jan Mohr in 1968 and by other researchers in the early 1970s. In 1982, a Soviet group reported CVS with ultrasound guidance; and the University College Hospital of London developed the now widely used technique of transcervical CVS guided by real-time ultrasound which can be performed much earlier than amniocentesis (at 9-14 weeks). Higher incidence of limb defects is reported when performed earlier than nine weeks, and risk of miscarriage is one in 100.
Maternal serum alpha-fetoprotein (MSAFP) measures the levels of AFP—a protein produced by the fetal liver--in the mother's blood. High levels can indicate a longer pregnancy than calculated, twins, spina bifida, or anencephaly (incomplete development of the skull or brain). Low levels can indicate a shorter time pregnant than calculated, or other problems such as Down's syndrome. These varying indications dictate further tests--such as ultrasound or amniocentesis.
Fetal blood sampling for diagnostic purposes has been done since 1972. A fiber optic endoscope inserted through an incision in the uterine wall which allows observation of the placenta, fetus, and amniotic fluid, as well as sampling of fetal, placental, or umbilical cord blood or tissue for diagnosis of hereditary blood or skin disorders.
This radical, high-risk procedure is used only in exceptional circumstances. DNA analysis for specific gene disorders was introduced to prenatal testing in 1976. Fetal samples from amniocentesis, CVS, and maternal and paternal blood samples are examined. More than 260 gene defects can now be diagnosed through DNA analysis, and the list grows. In 1982, Fernand Daffos and his team used real-time ultrasound imaging for cordoncentesis, or percutaneous umbilical blood sampling (PUBS), in which a hollow needle passes through the uterine wall collects blood from a vessel in the umbilical chord allowing chromosome count; measurement of oxygen, carbon dioxide, and bicarbonate if fetus growth retardation is suspected; and detection of fetal anemia and infections such as rubella and herpes.
In August, 1997, The Lancet reported that researchers from Britain, Italy, and Hong Kong discovered samples of fetal DNA in maternal blood plasma. This non-invasive test holds promise of a safer, more effective method of genetic testing. Bart's triple test, a new test which also screens maternal blood for AFP, human chorionic gonadotropin, and unconjugated estriol, helps determine the need for amniocentesis.
Pre-implantation genetic diagnosis (PGD), in which a woman's eggs are fertilized in vitro with her partner's sperm, allows earliest detection of fetal deformity thus far. One cell removed from the 8-cell fetus in the petri dish is checked for genetic defect before implantation into the uterus. The first PGD baby was born in 1989 and, by 1997, more than 30 babies had been born worldwide using this technique. Investigational as of 1998, a specialized test--the InSight Prenatal Test--uses flourescent DNA probes on amniotic fluid to detect, with a high degree of accuracy, the most common chromosome abnormalities. Prenatal diagnosis is now moving toward prenatal treatment: A few conditions--including respiratory distress syndrome, a prime killer of prematurely born infants, and a rare vitamin metabolism defect called methylamalonic acidemia--can be treated in the womb by medicating the mother, and prenatal surgery on the fetus is in its infancy.
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