Chromosomal abnormalities- Aneuploidy.pptx

Screening and diagnosis of aneuplodies in 1st and 2nd trimesters

1. Chromosomal abnormalities- Aneuploidy

2. Introduction Chromosomal abnormalities are produced when the normal 23 pairs of chromosomes that produces a karyotype are altered. They usually occur during cell division either by increase or decrease in the number of chromosomes. Some of these abnormalities are compatible with life, others are not. The role of healthcare professionals is to provide counselling, screening and diagnostics tests to all pregnant women, enabling couples to make informed decisions regarding their pregnancies.

3. Classification

4. Numerical chromosomal abnormalities Chromosomal abnormality Genetic syndrome Incidence per 10,000 births Trisomy 21 Down’s syndrome 15 Trisomy 18 Edward’s syndrome 3 Trisomy 13 Patau’s syndrome 2 Monosomy X Turner’s syndrome 2 XXY Klinefelter’s syndrome 10

5. Screening : first trimester Carried out between 11 weeks – 13+6 weeks of gestation. ( CRL- 45-84nm) Maternal age Nuchal translucency Maternal serum beta HCG levels Maternal serum PAPP-A levels Additional ultrasound markers Useful in prediction of preterm birth, pre eclampsia, gestational diabetes, still birth, FGR and macrosomia

6. Nuchal translucency NT is the measure of fluid in the neck region. It is found to be increased in foetuses with chromosomal abnormalities, congenital heart disease, exomphalos, congenital diaphragmatic hernia. Screening by NT can detect about 80% of fetuses with trisomy 21 and other major aneuploidies for a false positive rate of 5%. When combined with biochemical markers, the detection rate increases. T he gestational period must be 11 to 13 weeks and six days. The fetal crown-rump length should be between 45 and 84mm. The magnification of the image should be such that the fetal head and thorax occupy the whole screen. A mid-sagittal view of the face should be obtained. This is defined by the presence of the echogenic tip of the nose and rectangular shape of the palate anteriorly, the translucent diencephalon in the centre and the nuchal membrane posteriorly. The fetus should be in a neutral position, with the head in line with the spine. Callipers should be placed on the inner borders of the white lines. The widest part of translucency must always be measured . The measurement should be repeated and the maximum reading is used.

7. Biochemical markers: beta HCG Beta- HCG is produced by the human placenta It peaks at 15 weeks, following which there is a rapid decline till 17 weeks, a gradual fall till 22 week. Increased in foetuses with trisomy 21 from the end of first trimester throughout the second trimester. It is decreased in other trisomies . In women with HIV, Beta HCG levels are found to be lower, hence results might be falsely negative.

8. Biochemical markers: PAPP-a Pregnancy associated plasma protein A is also produced by the placenta. It is reduced in majority of the aneuplodies during the first trimester It is a useful marker only upto 14 weeks of gestation. It is suggested that maternal serum PAPP- A is measured at 9 weeks, followed by perfoming an NT scan and beta- HCG levels at 12 weeks. This increases the sensitivity of screening to 95%.

9. Ultrasound markers Absence of nasal bone Reversed a wave in ductus venosus Tricuspid regurgitation Frontomaxillary facial angle Structural abnormalities- AVS defects, exomphalos, holoprosencephaly, megacytitis .

10. Screening: Second trimester It is carried out between 15-21 weeks of gestation It involves a detailed ultrasound at 20- 22 weeks Biochemical markers- Beta HCG, Inhibin A, AFP and uE3 levels Triple and Quadruple tests

11. Biochemical markers: alpha fetoprotein Produced by the liver and GIT of the foetus and excreted into the amniotic fluid. Was initially identified as a marker of open spina bifida It is found to be reduced in trisomy 21. It is elevated in Neural tube defects, GIT disorders, FGR, Pre eclampsia, renal anomalies, osteogenesis imperfecta, oligohydramnios, abruptio placenta and placental villous thrombosis.

12. Biochemical markers: free estriol Free estriol (uE3) is a product of the breakdown of dehydroepiandrosterone sulphate which is produced by the fetal adrenal glands. Decreased in pregnancies with trisomy 21 If less than 0.25 MoM- Smith-Lemli-Opitz syndrome & steroid sulfatase deficiency.

13. Biochemical markers: Inhibin a In non pregnant women inhibin A is produced by the corpus luteum. The levels are markedly elevated during pregnancy as it is produced by the placenta and inhibits FSH First trimester levels are similar in affected & unaffected pregnancies It is increased to about 1.77 MoM in the second trimester in trisomy 21.

14. Comprehensive ultrasound It is carried out between 20-22 weeks of gestation Presence of one or more soft markers suggests an increased risk of chromosomal anomalies. SOFT MARKERS IN USG Aberrant subclavian artery Echogenic bowel Echogenic cardiac foci Absent nasal bone or hypoplasia Renal pelvis dilatation Single umbilical artery Shortened long bones

15. Screening methods & their detection rates Screening method Detection rate False positive Maternal age 30 5 NT alone 77 4.7 Double test- AFP +beta HCG 58 5 Triple test- AFP + beta HCG + uE3 69 5 Quadruple test- triple + Inhibin A 85 5 Combined test – maternal age + NT+ PAPP- A + beta HCG 85-90 5 Integrated test- NT + PAPP- A + quad in 2 nd trimester 85 1.2

16. Diagnostic tests Diagnostic testing will either confirm or exclude a suspected diagnosis Counselling is a prerequisite to diagnostic tests due to the associated risks First trimester- chorionic villus sampling Second trimester- amniocentesis

17. Chorionic villus sampling Performed between 11- 14 weeks of gestation Cells are obtained from the chorion for karyotyping There are 2 approaches- transabdominal and transcervical Risks- foetal loss, cramping & spotting, chorioamnionitis, rupture of membranes, isoimmunisation in rh negative mothers,

18. amniocentesis Performed after 16 weeks The foetal skin cells obtained from the amniotic fluid are cultured for karyotyping It should be done under ultrasound guidance with a 20 or 22 gauge needle. 1-3 ml is discarded and 20 – 30 ml fluid is aspirated for testing Methods used for analysis- karyotyping, Fluorescence in situ hybridisation (FISH) and Chromosomal microarray (CMA) Early amniocentesis is performed at 11- 14 weeks and less fluid (1 ml) is aspirated. It is associated with higher risk

19. cordocentesis Also known as fetal blood sampling or percutaneous umbilical cord blood sampling Under USG guidance with a 22 or 23 gauge needle, blood is slowly drawn with a heparinised near the site of umbilical cord insertion. Alternatively, a free loop of cord maybe punctured The complications include fetal loss, fetal bradycardia, fetomaternal haemorrhage and visible bleeding from the cord,

20. Non invasive prenatal testing The test works by identifying DNA fragments in maternal serum that are derived from apoptotic trophoblasts- cell free DNA Can be performed between 9-10 weeks of gestation Assays available include genome sequencing, chromosome selective and single nucleotide polymorphism analysis Its use in second trimester is limited, hence performed only when first trimester screening shows a high risk. May indicate confined placental mosaicism, early demise of twin aneuploidy fetus , occult maternal cancers Not recommended in twin gestations.

21. Preimplantation genetic testing In couple undergoing IVF, it is useful to perform genetic testing on oocytes or embryos before implantation. Both screening and diagnostic tests are available. Polar body analysis- the oocyte is affected by maternal inherited genetic disorders. Blastomere biopsy- done at the 6-8 celled stage. Both maternal and paternal genomes can be evaluated. It however cannot identify mosaic pattern of disease. Trophectoderm biopsy- done at 5-7 celled stage. Tests the trophoblast cells rather than cells from the embryo.

22. Acog guidelines Prenatal genetic screening and diagnostic testing options should be discussed and offered to all pregnant patients regardless of maternal age or risk of chromosomal abnormality. If screening is accepted, patients should have one prenatal screening approach, and should not have multiple screening tests performed simultaneously. Cell-free DNA is the most sensitive and specific screening test for the common fetal aneuploidies. It has the potential for false-positive and false-negative results. Furthermore, cell-free DNA testing is not equivalent to diagnostic testing. All patients should be offered a second-trimester ultrasound for fetal structural defects. Ideally this procedure is performed between 18 and 22 weeks of gestation (with or without second‐trimester maternal serum alpha‐fetoprotein). Patients with a positive screening test result for fetal aneuploidy should undergo genetic counseling and a comprehensive ultrasound evaluation with an opportunity for diagnostic testing to confirm results. Patients with a negative screening test result should be made aware that this substantially decreases their risk of the targeted aneuploidy but does not ensure that the fetus is unaffected. If an enlarged nuchal translucency or an anomaly is identified on ultrasound examination, the patient should be offered genetic counseling and diagnostic testing for genetic conditions and a comprehensive ultrasound evaluation including detailed ultrasonography at 18–22 weeks of gestation to assess for structural abnormalities.

23. Acog guidelines The use of cell-free DNA screening as follow-up for patients with a screen positive serum analyte screening test result is an option for patients who want to avoid a diagnostic test. In clinical situations of an isolated soft ultrasonographic marker. The patient should be counseled regarding the risk of aneuploidy associated with the finding and cell-free DNA, quad screen testing, or amniocentesis should be offered. If aneuploidy testing is performed and is low risk, then no further risk assessment is needed. If more than one marker is identified, then genetic counseling, maternal–fetal medicine consultation, or both are recommended. No method of aneuploidy screening that includes a serum sample is as accurate in twin gestations as it is in singleton pregnancies. Cell-free DNA screening can be performed in twin pregnancies. Overall, performance of screening for trisomy 21 by cell-free DNA in twin pregnancies is encouraging.. The use of multiple serum screening approaches performed independently ( eg , a first-trimester screening test followed by a quad screen as an unlinked test) is not recommended because it will result in an unacceptably high positive screening rate and could deliver contradictory risk estimates. In multifetal gestations, if a fetal demise, vanishing twin, or anomaly is identified in one fetus, there is a significant risk of an inaccurate test result if serum-based aneuploidy screening or cell-free DNA is used. This information should be reviewed with the patient and diagnostic testing should be offered. Patients with unusual or multiple aneuploidies detected by cell-free DNA should be referred for genetic counseling and maternal–fetal medicine consultation.