FIRST-TRIMESTER SCREENING :

First-trimester screening by means of maternal age and measurement of nuchal translucency could provide a trisomy 21 detection rate of 63 percent, with a 5 percent false-positive rate.

Combining this with measurement of maternal serum free beta-hCG subunit and pregnancy associated protein A could increase the detection rate to 80 percent, at the same false- positive rate.

RECURRENCE RISK AND FAMILY HISTORY
If a patient has had a trisomy 21 pregnancy in the past, the risk of recurrence in a subsequent pregnancy is still highly dependent on maternal age. If the previously affected pregnancy was a chromosome-21 translocation the recurrence risk is high enough to dwarf the age- specific risk at most ages.

PRENATAL DIAGNOSIS :

Definitive prenatal diagnosis of trisomy 21 requires cytogenetic analysis of cells obtained by one of the three invasive procedures:

• Chorionic villus sampling
  
  
• Early amniocentesis
  
  
• Second-trimester amniocentesis
  
  

Chorionic villus sampling (CVS) and early amniocentesis are the primary diagnostic procedures used between 11 and 14 weeks gestation.

ADVERSE EFFECTS OF SCREENING AND EARLY DETECTION

The most important risks of early detection of Down syndrome include:

• To the fetus from amniocentesis and CVS performed as a primary or follow- up diagnostic test:
  
  The risks of amniocentesis include
  
  

  • Rare puncture of the fetus
  • BleedingInfectionIsosensitization
  • Neonatal respiratory distress syndrome
  • Neonatal pneumonia

  
  The risk of CVS include
  
  

  • Limb reduction defects
  • Bleeding

  
  

However if CVS is done at 10 weeks or later the incidence of fetal defects is reduced.

• The psychological effects of a positive test on the parents:

A positive screening test result can produce a harmful psychological effect on parents. This is important because the large majority of positive screening tests occur in normal pregnancies Adverse psychological effects of screening tests include the fear of discovering an abnormal pregnancy as well as anxiety over possible complications from diagnostic and therapeutic procedures. Women who have been identified as being at high risk because of positive serum marker screening test may have greater distress than women who are identified as high risk because of advanced age.(13, 14)

• Complications resulting from induced abortion:
  
  

The potential complications of induced abortion must also be considered, since this is the outcome of the majority of positive diagnostic test results.

• Ethical issues:
  
  

Critical ethical issues are raised by selective abortion for DS pregnancies. Society may interpret the offer of diagnosis and termination for DS fetuses as DS, an undesirable state and DS individuals to be worthless.