Lhdlhdkhd

1. Prenatal screening
Tinsae Genet, OBGY R3
Moderator: Dr. Yeshiwas Abebaw,
Assistant Professor of OBGY, UOG.
10/13/2023 prenatal screening 1

2. Objective
• Explain basic prenatal testing methods of common congenital
malformations and genetic disorders by which we can adopt to our
setup based on the facility and human power we have.
10/13/2023 prenatal screening 2

3. Session outline
• Definition and objectives of prenatal screening and diagnosis
• Prenatal screening and diagnosis of neural tube defect
• Prenatal screening and diagnosis of aneuploidies
• Highlights on preimplantation genetic tests
10/13/2023 prenatal screening 3

4. Defn.
• Is the science of identifying malformations, disruptions,
chromosomal abnormalities, and other genetic syndromes in the
fetus.
• Detecting or defining risk for disease in an asymptomatic low risk
population is the goal of screening.
• Diagnostic testing is intended to identify or confirm an affected
individual.
10/13/2023 prenatal screening 4

5. • It encompasses;
• Routine screening tests for aneuploidy and neural-tube defects,
• Invasive diagnostic tests such as chorionic villus sampling and amniocentesis,
• Additional screening and diagnostic tests offered to those at risk for specific
genetic disorders, and
• The diagnosis of structural malformations with specialized sonography and
other fetal imaging techniques.
10/13/2023 prenatal screening 5

6. Goal
• To provide accurate information regarding short- and long-term
prognosis,
• Recurrence risk, and
• Potential therapy and to thereby improve counseling and optimize
outcomes.
10/13/2023 prenatal screening 6

7. An ideal perinatal genetic screening test;
• Identify common or important fetal disorders
• Be cost-effective and easy to perform
• Have a high detection rate and a low false-positive rate
• Be reliable and reproducible
• Screen for disorders for which a diagnostic test exists
• Be positive early enough in gestation to permit safe and legal options
for pregnancy termination if desired
10/13/2023 prenatal screening 7

8. NEURAL TUBE DEFECTS (NTD)
• These defects include
anencephaly,
spina bifida,
cephalocele, and
other rare spinal fusion (schisis) abnormalities
• Reported frequency is approximately 0.9 per 1000 births.
• Ethiopia: overall NTD prevalence of 6.1/1000.
10/13/2023 prenatal screening 8

9. NTD cont.
• Alpha-fetoprotein (AFP) level was first maternal serum screening test
for a fetal defect.
• Recurrence risks:
3 to 5% if a couple has previously had a child with either anencephaly or
spina bifida
5% if either parent was born with a NTD
As high as 10% if a couple has two affected children
10/13/2023 prenatal screening 9

10. Risk factors
Multifactorial  Genetics + Environmental
• Polymorphisms in the methylene tetrahydrofolate reductase gene,
• Hyperthermia,
• Medications that disturb folic acid metabolism, and
• Hyperglycemia from insulin-dependent diabetes
• Certain racial or ethnic groups
10/13/2023 prenatal screening 10

11. Prevention
• Most women at increased risk for NTDs benefit from 4 mg folic acid
taken daily before conception and through the first trimester.
• This is particularly important
a woman has one or more prior affected children or
if either the pregnant woman or her partner has such a defect.
• Routine fortification of cereal grains with folic acid may reduce the
first occurrence of NTDs in low-risk women by approximately 20%.
400 μg of folic acid orally every day before conception and through
the first trimester, to reduce the NTD risk by as much as 80%.
10/13/2023 prenatal screening 11

12. screening
candidates
• All pregnant women should be offered screening for NTDs. (ACOG 2013b)
• Once the gestational age is verified and the screening test is confirmed
to be abnormal, a patient is offered diagnostic evaluation.
• Some women have risk factors that warrant a diagnostic test even in
the setting of a normal AFP level.
Personal history of or first-degree relative with a NTD,
Insulin-treated diabetes, and
First-trimester exposure to a medication associated with increased risk.
10/13/2023 prenatal screening 12

13. Screening cont.
Screening tests:
Ultrasonography – primary screening tool
Maternal serum AFP
The evaluation begins with a standard sonogram.
• Reliably exclude three common causes of AFP level elevation:
underestimation of gestational age,
multifetal gestation, and
fetal demise.
10/13/2023 prenatal screening 13

14. Screening cont.
• Maternal serum AFP screening is generally performed from 15 – 20 wks.
• Measured in nanograms per milliliter and reported as multiples of the
median (MoM) of the unaffected population.
• Using a maternal serum AFP level of 2.0 or 2.5 MoM as the upper limit
of normal, most laboratories report a detection rate, sensitivity of 90%
for anencephaly and 80% for spina bifida at a screen-positive rate of
3 – 5%.
10/13/2023 prenatal screening 14

15. 10/13/2023 prenatal screening 15

16. Screening cont.
• Several factors influence maternal serum AFP levels & interpretations
1. Maternal weight
2. Gestational age
3. Race/ethnicity and geography
4. Diabetes
5. Multifetal gestation
10/13/2023 prenatal screening 16

17. 10/13/2023 prenatal screening 17

18. • Women with a confirmed serum AFP level elevation should be
referred for additional counseling and offered a diagnostic test,
either
specialized sonography or
Amniocentesis
• Most centers use targeted sonography as the primary method of
evaluating maternal serum AFP level elevation.
• ACOG (2013b) recommends that women be counseled regarding
the risks and benefits of targeted sonography and amniocentesis.
10/13/2023 prenatal screening 18

19. 10/13/2023 prenatal screening 19

20. Targeted Sonography
• 99% of fetuses with open spina bifida had one or more of these
findings in second-trimester
― frontal bone scalloping— the lemon sign,
― anterior curvature of the cerebellum with effacement of the cisterna
magna— the banana sign
― small biparietal diameter
― ventriculomegaly.
• Transverse and sagittal images of the spine: used to characterize the
size and location of spinal defects with 100% detection of open
NTDs.
• Overall NTD risk may be reduced by at least 95% when no spine or
cranial abnormality is observed.
10/13/2023 prenatal screening 20

21. • Anencephaly should be routinely identified by ultrasound at as early
as 11 or 12 weeks’ gestation
• Should be reconfirmed by a scan at around 13 – 14 weeks.
• Lumbo sacral myelomeningocel
• Anencephaly
10/13/2023 prenatal screening 21

22. The “lemon” sign
10/13/2023 prenatal screening 22

23. Amniocentesis
• If the amnionic fluid AFP level is elevated, an assay for acetyl
cholinesterase is performed, and if positive, was considered
diagnostic of a NTD.
• The most common assay for AChE is a polyacrylamide gel
electrophoresis.
• Acetyl cholinesterase leaks directly from exposed neural tissue into
the amnionic fluid.
• A combined use of AFP and AChE together appears to be the most
sensitive and specific in determining neural tube defects.
• Overall sensitivity = 98% for open NTDs, with a false-positive rate of
0.4%.
10/13/2023 prenatal screening 23

24. Prenatal screening and Diagnosis of
Aneuploidies
• Down syndrome is the commonest.
• At least 8% of conceptuses are aneuploid,
50% of first-trimester abortions and
5 to 7% of all stillbirths and neonatal deaths.
• Moderate to severe learning disability, heart
defects, intestinal malformations, vision and
hearing loss.
• High financial and psychological support to
the family
10/13/2023 prenatal screening 24

25. Risk factors for aneuploidies
• The risk of fetal trisomy increases with maternal age, particularly after
age 35.
• Other significant risk factors include
a prior pregnancy with autosomal trisomy or triploidy or
a woman or her partner with a numerical chromosomal abnormality or
structural chromosomal rearrangement, such as a balanced translocation.
10/13/2023 prenatal screening 25

26. Candidates
• All women who present for prenatal care before 20 weeks be
offered screening. (ACOG 2013c)
• Positive screening test  increased risk, but not diagnostic of
aneuploidy.
• Negative screening test risk not increased, but does not guarantee
a normal fetus.
• Regardless of age, all women are counseled regarding the differences
between screening and diagnostic tests, and they are given the option
of invasive diagnostic testing.
10/13/2023 prenatal screening 26

27. Prenatal screening and Diagnosis
of Aneuploidies cont.
screening tests
• 1st Trimester
 Ultrasound
 Maternal serum markers
• 2nd trimester
 Triple test
Quadruple test
Maternal serum free cell fetal DNA
• Combined 1st & 2nd Trimester Screening
Integrated screening
Sequential screening
stepwise sequential screening
contingent sequential screening
Diagnostic tests
• Invasive procedure
Chorionic villi sampling
Amniocentesis
Fetal blood sampling
• Preimplantation diagnostic
testing
10/13/2023 prenatal screening 27

28. First-Trimester Screening
• This is performed between 11 and 14 weeks of gestation.
• The most commonly used protocol involves:
measurement of sonographic nuchal translucency
two maternal serum analytes.
10/13/2023 prenatal screening 28

29. Nuchal Translucency (NT)
• Approximately one third of fetuses with increased NT thickness will have a
chromosome abnormality, nearly half of which are Down syndrome.
• NT detects 64 – 70% of fetuses with Down syndrome at a false positive
rate of 5%.
• Has maximal sensitivity at 11 weeks.
• Combining NT with serum analyte values  greatly improved aneuploidy
detection.
• Exceptions:
• Multiple gestation
• NT thickness > 3 – 4mm
10/13/2023 prenatal screening 29

30. •Nuchal translucency
10/13/2023 prenatal screening 30

31. Serum Analytes
• Two analytes are used for first-trimester aneuploidy screening:
Human chorionic gonadotropin—either total or free β-hCG
Pregnancy-associated plasma protein A (PAPP-A)
• In cases of fetal Down syndrome,
the 1st TM serum free β-hCG level is higher  2.0 MoM,
the PAPP-A level is lower  0.5 MoM.
• With trisomy 18 and trisomy 13, levels of both analytes are lower.
• Detection rates for fetal Down syndrome is up to 67% alone at a
false-positive rate of 5%.
10/13/2023 prenatal screening 31

32. Combined First-Trimester Screening
• The most commonly used screening protocol.
• Down syndrome detection rates  79 – 87%, at a false-positive rate
of 5%.
• Detection rate for trisomies 18 and 13  90%, at a 2% false-positive
rate.
• Best detection rate  at 11 week.
10/13/2023 prenatal screening 32

33. Unexplained Abnormalities of 1st TM Analytes
• There is a significant association between serum PAPP-A levels below
the 5th percentile and preterm birth, growth restriction,
preeclampsia, and fetal demise.
• Similarly, low levels of free β-hCG have been associated with fetal
demise.
10/13/2023 prenatal screening 33

34. Second Trimester Screening
• The Quad test is the most commonly used second-trimester serum
screening test for aneuploidy.
• As a stand-alone test, it is generally used if:
 women do not begin care until the 2nd TM or
first-trimester screening is not available.
• Also done as part of
Integrated test
Sequential test
• Triple test can also be used with detection rate of 61 – 70% of Down
syndrome cases.
10/13/2023 prenatal screening 34

35. Pregnancies with fetal Down syndrome are characterized by:
lower maternal serum AFP levels
higher hCG levels triple test
lower unconjugated estriol levels
• Levels of all three markers are decreased in the setting of trisomy 18.
• Levels of a fourth marker — dimeric inhibin alpha  elevated in
Down syndrome, with an average value of 1.8 MoM QUAD TEST
Detection rate of approximately 80% at a false-positive rate of 5%.
10/13/2023 prenatal screening 35

36. Abnormal serum analytes Pregnancies with fetal
Down syndrome in MoM
10/13/2023 prenatal screening 36

37. 10/13/2023 prenatal screening 37

38. Unexplained Abnormalities of 2nd Trimester
Analytes
• Likelihood of adverse outcome increased when multiple marker levels
are elevated like:
• fetal growth restriction
• preeclampsia
• preterm birth
• fetal demise
• However, the sensitivity and positive predictive values of these
markers are considered too low to be useful for screening or
management.
10/13/2023 prenatal screening 38

39. Cell-Free Fetal DNA Screening
• Using massively parallel sequencing or chromosome selective
sequencing to isolate cell-free fetal DNA from maternal plasma.
• In high-risk pregnancies  detection rates for trisomies 21, 18, and
13 = 98% at a false-positive rate of 0.5% or less (ACOG, 2012b; and
others).
• Timing: 12 – 20 wks.
• Limitation: uninformative, expensive.
10/13/2023 prenatal screening 39

40. Candidates
• ACOG(2012b) currently recommends that the test may be offered to
the following groups:
1. Women 35 years or older at delivery
2. Those with sonographic findings indicating increased risk for fetal
aneuploidy
3. Those with a prior pregnancy complicated by trisomy 21, 18, or 13
4. Patient or partner carries a balanced robertsonian translocation indicating
increased risk for fetal trisomy 21 or 13.
5. Those with an abnormal first-, second-, or combined first and second-
trimester screening test result for aneuploidy.
10/13/2023 prenatal screening 40

41. Sensitivity of different prenatal screening strategies
10/13/2023 prenatal screening 41

42. Sonographic Screening
Soft Signs:
• Are minor markers that may improve sonographic detection of
aneuploidy, particularly Down syndrome.
• Are normal variants.
• Used in 15 – 22 wks of gestation.
• Risk of aneuploidy increases steeply with the number of markers
identified.
Their incorporation into 2nd TM screening protocols has been studied
primarily in high-risk populations  detection rates of 50 to 75% for
Down syndrome have been reported (ACOG, 2013c).
10/13/2023 prenatal screening 42

43. Soft Signs cont.
• Are many, but Six of these markers have been the focus of genetic
sonogram studies.
1. Nuchal skin fold thickening
2. Echogenic intracardiac focus
3. Pyelectasis
4. Echogenic bowel
5. Clinodactyly
6. Sandal gap
10/13/2023 prenatal screening 43

44. Nuchal fold
• By transcerebellar view of fetal head
from the outer edge of skull to the
outer skin
• Measure ≥ 6mm  abnormal with
>10 fold increase risk of down
syndrome
• Amniocentesis should be performed
even if found as an isolated finding
10/13/2023 prenatal screening 44

45. Echogenic intracardiac focus
• A focal intracardiac papillary
calcification
• Not a cardiac abnormality
• Usually left sided
• Double the risk of down
syndrome if found isolated
• Common in trisomy 13
10/13/2023 prenatal screening 45

46. Pyelectasis
• Transient or physiologic finding in
normal fetus
• Measured in transverse image of the
pelvis
• Size ≥ 2mm in 2% of fetuses
• Double risk of down syndrome
• Further additional testing for having
renal abnormality should be
performed at GA 34 wks.
10/13/2023 prenatal screening 46

47. Echogenic fetal bowel
• Six fold increase risk of down
syndrome
• Appears as bright as bone
• Represents:
Small amount of swallowed blood
Fetal CMV infection
Fetus with cystic fibrosis
10/13/2023 prenatal screening 47

48. Sandal gap
Clinodactyly
• Other soft signs: short femur and humeral length, single umb. artery,
widened iliac angle, brachycephaly, flat facies, absent or hypoplastic nasal
bone
• Hypoplasia of the fifth finger middle
phalanx
10/13/2023 prenatal screening 48

49. Prenatal diagnostic tests for aneuploidy
• Are invasive.
• Include:
 Chorionic villi sampling
 Amniocentesis
 Fetal blood sampling
10/13/2023 prenatal screening 49

50. Amniocentesis
• Most common procedure
• Used to diagnose fetal genetic conditions  karyotyping
• Done at 15 – 20 wks of gestation.
• Takes 7 – 10 days to culture the amniocytes and the assessment of
karyotype.
10/13/2023 prenatal screening 50

51. Technique
• Sonographic guidance
• Use 20 – 22 gauge spinal needle
• Amniotic fluid should be clear.
• Initial 1 – 2ml of fluid is contaminated , so
discard and collect 20ml of amniotic fluid
• After removing of needle, puncture site is
observed for bleeding and fetal cardiac
motion is documented
• RH negativity is a relative contraindication.
Administer anti D and continue.
• Multiple gestation: use indigo carmine
10/13/2023 prenatal screening 51

52. Typical volume required for selected testes
10/13/2023 prenatal screening 52

53. Amniocentesis cont.
Complication:
• Pregnancy loss: 1 in 300 – 500.
Doubled if maternal BMI ≥ 40 kg/m2
• Amniotic fluid leakage: usually in the 1st 24 hr.
• Chorioamnionitis
10/13/2023 prenatal screening 53

54. Chorionic Villi Sampling (CVS)
• Done between 10 – 13 wks of gestation
• For karyotyping and specialized genetic testing
• Results are earlier allowing safer pregnancy termination
• Result available in 7 – 10 days
10/13/2023 prenatal screening 54

55. Technique
• Can be either
Transabdominal
Transcervical
• Transcervical: blunt tipped special
catheter with malleable stylet
• Transabdominal one: by 18 – 20
gauge spinal needle
• Transabdominal ultrasound is
used to guide the needles.
10/13/2023 prenatal screening 55

56. CVS cont.
Complication;
• Fetal loss rate = 2% compared with <1% following amniocentesis.
• An early CVS is also associated with limb reduction defect and
oromandibular hypogensis, if GA < 7 wk.
• Vaginal spotting Is common following transcervical sampling  self
limited and not associated with pregnancy loss.
• Chorioamnionitis in <0.5% of cases.
10/13/2023 prenatal screening 56

57. Fetal blood sampling
• Cordocentesis or percutaneous umbilical blood sampling (PUBS)
• Initially described for fetal transfusion of RBC.
• Still fetal anemia assessment is the most common indication
• Can also be done for fetal karyotyping – result within 24 – 48 hr.
10/13/2023 prenatal screening 57

58. Technique
• Under direct sonographic guidance
• Using a 22 – 24 gauge spinal needle
into umbilical vein and blood is slowly
withdrawn into a heparinized syringe.
• Often performed near the placenta.
• Local anesthetic
• Prophylactic antibiotics?
• After needle removed – follow fetal
cardiac activity and the site of
insertion for bleeding
10/13/2023 prenatal screening 58

59. Fetal blood sampling
• Fetal loss rate = 1.4%
• Other complications
Cord vessel bleeding = 20 – 30% of cases
Fetomaternal bleeding = 40% of cases
Fetal bradycardia = 5 – 10%
Most complications are transitory with complete recovery
10/13/2023 prenatal screening 59

60. Preimplantation genetic testing
• Genetic testing performed on oocytes or embryos before
implantation or invitro fertilization
• May provide valuable information regarding the chromosomal
complement and single gene disorders
• Includes
Preimplantation genetic diagnosis
Polar body analysis
Blastomere biopsy
Trophectoderm biopsy
Preimplantation genetic screening
PCR – amplified genomic region
FISH – chromosome rearrangement
N.B. Must be accompanied by CVS or
amniocentesis to confirm it.
10/13/2023 prenatal screening 60

61. References
• Gabbe OBSTETRICS NORMAL AND PROBLEM PREGNANCIES 6th Edition
• Williams Obstetrics 24th edition
• Counseling about genetic testing and communication of genetic test
results, ACOG committee opinion, number 693, April 2017.
• Up-To-Date 21.6
• Prenatal diagnostic testing for genetic disorders, practice bulletin 162, May
2016.
• Prenatal Screening, Diagnosis, and Pregnancy Management of Fetal Neural
Tube Defects, SOGC, clinical practice guideline, No. 314, October 2014
• PATTERNS OF NEURAL TUBE DEFECTS AT TWO TEACHING HOSPITALS IN
ADDIS ABABA, ETHIOPIA A THREE YEARS RETROSPECTIVE STUDY, Sorri
G, Mesfin E, Ethiop Med J. 2015 Jul;53(3):119-26
10/13/2023 prenatal screening 61

62. 10/13/2023 prenatal screening 62