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Screening options

Screening for Down syndrome
1)  If patient presents before 14 weeks:
Tests
Down syndrome DR
Down syndrome FPR
Comments
Integrated Prenatal Screening (IPS)
First Trimester (11-13+6/7wks)
– ↑NT – by registered sonographer
– maternal serum: ↓PAPP-A
Second Trimester (15-20+6/7wks)
– maternal serum: ↓AFP, ↑hCG, ↓uE3
85-90%
2-4%
– Results available in 2nd trimester after blood taken
– Diagnostic test after counselling for screen positive = amniocentesis
Serum Integrated Prenatal Screening (SIPS)
First Trimester (11-13+6/7wks)
– maternal serum: ↓PAPP-A
Second Trimester (15-20+6/7wks)
– maternal serum: ↓AFP, ↑hCG, ↓uE3, ↑DIA
80-90%
2-7%
– Results available in 2nd trimester after blood taken
– Diagnostic test after counselling for screen positive = amniocentesis
– Is available in most places where NT ultrasound is not available
(*) First Trimester Combined Screening (FTS)
First Trimester (11-13+6/7wks)
– ↑NT – by registered sonographer
– maternal serum: ↓PAPP-A, ↑ƒbhCG
78-85%
3-9%
– Results available in 1st trimester after blood taken – usually end of 1st trimester,

earliest results

– CVS for diagnostic testing
Does not screen for NTD*
(*) First trimester screening is not available in all areas of Ontario
* NTDs (open neural tube defects) can be screened for by MS-AFP and/or ultrasound at 18-20 weeks
DR: detection rate – also known as sensitivity, is the probability that a fetus affected with Down syndrome will be detected by the prenatal test.
FPR: false positive rate – the proportion of women with unaffected pregnancies who have positive results.
↑: increased value
↓: decreased value
2)  If patient presents after 14 weeks:
Test
Down syndrome DR
Down syndrome FPR
Comments
Maternal Serum Screen (Quadruple Screening – MSS)
Second Trimester (15-20+6/7 wks)
– maternal serum: ↓AFP, ↑hCG, ↓uE3, ↑DIA
75-85%
5-10%
– Results available in 2nd trimester
– Amniocentesis for diagnostic testing
Screening for Open Neural Tube Defects and Trisomy 18
Open Neural Tube Defects
Trisomy 18
Maternal serum
↑AFP
↑NT, ↓PAPP, ↓ƒbhCG ↓AFP, ↓hCG, ↓uE3, ↓DIA
DR
80% for each test except FTS which does not screen for NTDs
Slightly lower than the DR for Down syndrome for each test
FPR
Usually 5% or less for all tests except FTS
Lower than the FPR for Down syndrome for each test.  Usually 1% or less.
Pregnancy dating: Ultrasound dating is more accurate than LMP, a dating ultrasound will lower the FPR.
Listed in the table below are prenatal ultrasound findings that could be associated with Open Neural Tube Defects, Trisomy 18 and Down syndrome.  It is important to note that soft markers identified on ultrasound increase the likelihood of a chromosome problem being present but can also be seen in normal pregnancies.

Condition

Ultrasound Findings

Open Neural Tube Defects

ventriculomegaly, chiari malformation type II, club feet

Trisomy 18
↑NT, microcephaly, micrognathia, cardiac defect, limb abnormality, overlapping fingers, rocker bottom feet, choroid plexus cyst
Down syndrome↑NT, echogenic intracardiac focus, absent nasal bone, short femur and humerus length, cardiac defect, echogenic bowel, duodenal atresia, mild ventriculomegaly
References
Joint SOGC-CCMG Clincial Practice Guideline: Prenatal Screening for Fetal Aneuploidy in Singleton Pregnancies. July 2011. JOGC 2061: 736-750.
SOGC Clinical Practice Guideline: Fetal Soft Markers in Obstetric Ultrasound. J Obstet Gynaecol Can 2005; 27(6): 592-612.
Sanders, R.C., Blackmon, L.R., Hogge, W.A., Spevak, P., Wulfsberg, E.A. 2002. Structural Fetal Abnormalities the Total Picture. Mosby Inc.