Genetics for UG nurses: prenatal testing

Genetics for UG nurses: prenatal testing

1. Prenatal testing and diagnosis

2. Introduction Prenatal testing is testing for diseases or conditions in a fetus or embryo before it is born. The aim is to detect birth defects, chromosome abnormalities, genetic diseases and other genetic conditions Screening can also be used for prenatal sex discernment. Common testing procedures include amniocentesis, ultrasonography, serum marker testing, or genetic screening. In some cases, the tests are administered to determine if the fetus will be aborted, so one can plan for appropriate care

3. Purposes To provide means to detect --- the defects that are best corrected soon after delivery Condition that may require preterm delivery for early correction Condition that may require cesarean delivery Conditions that may require medical or surgical treatment before birth Conditions on which a decision may be based to terminate a pregnancy

4. Screening test : test only indicates a higher risk than expected in the general population Diagnostic test : test determines with high degree of accuracy the presence or absence of a birth defect or genetic disorder

5. Indications 2-3% risk of major birth defect or genetic disorder with each pregnancy in general population But it is not indicated to all because of risk, cost and specificity of diagnostic procedure. Few important indications are Maternal age more than 35 at the time of delivery Elevated or low trisomy profile screen results Previous child with genetic defect Previous still birth or neonatal death Structural abnormality in mother and/or father

6. Balanced translocation Balanced translocation in mother or father Inherited disorders: cystic fibrosis, metabolic disorders, sex linked recessive disorder Medical disease in mother Exposure to teratogen Infection : TORCH group infections Abnormal ultrasound findings

7. Triple test: this test should be offered to high risk mothers for screening of genetic abnormalities in fetus Three chemicals; alpha-fetoprotein (AFP), human chorionic gonadotropin (HCG) and unconjugated estriol (uE3) are synthesized by fetal liver. These levels can be measured from maternal blood sample and compared with norms for that point of gestation Low AFP and uE3 levels and high HCG levels indicate high risk of down syndrome Elevated AFP levels indicate high risk for neural tube defect

8. 2. Ultrasonography It can be a screening or diagnostic technique, depending on sophistication of the equipment and skill of radiologist Ultrasound can potentially detect major or minor findings that may necessitate further diagnostic testing High resolution ultrasonography is useful in detecting birth defects like congenital heart defects, neural tube defects, kidney and bladder anomalies

9. 3. Amniocentesis It is one of diagnostic procedure with various purposes Time 14-16 weeks of gestation Ultrasound guided, clear tap of 20 cc fluid per abdomen The procedure carries 1-1.5% risk of abortion Fetal cells are cultured for chromosomal analysis, DNA may be extracted from fetal cell to detect single gene disorder. AFP levels in amniocentesis is reliable indicator of NTD Acetyl cholinesterase is found positive along with elevated AFP is diagnostic of an open NTD

10. 4. Chorionic villous sampling It is less commonly used procedure to obtain cells for chromosome analysis It is performed between 9-12 weeks of gestation either transcervically or transabdominally Thus if abnormality is detected, abortion is safely done Faster results are available But there is higher rate of fetal loss following procedure

11. 5. Feto scopy The procedure involves visualization of fetus using a fibreoptic self illuminated instrument called fetoscope It is inserted in amniotic cavity under local anesthesia Time -18-22 weeks It helps to detects malformation e.g. cleft lip and palate , obtain fetal blood sampling, skin biopsy

12. 6. Fetal blood sampling/ cordocentesis / percutaneous umbilical blood sampling Time- after 18 weeks FBS is needed to make prenatal diagnosis of sickle cell disease, hemophilia, Duchenne muscular dystrophy, immune deficiency disorder 10% risk of abortion 7. Fetal biopsy Diagnose certain genetic skin disorder when DNA studies are unavailable

13. 8. Pre implantation genetic diagnosis Through the technique of in vitro fertilization, the embryo can be tested at six to eight cell stage for presence of specific genetic disorder. If embryo found normal, then it would be implanted. There is significant false results because of contamination, thus adverse effect should be followed by CVS for confirmation

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