Because of the morbidity associated with Down syndrome, screening and diagnostic testing for this condition are offered as optional components of prenatal care. Prenatal diagnosis of trisomy 21 allows parents the choice of continuing or terminating an affected pregnancy.

The incidence of fetal trisomies is directly related to maternal age. The risk of having a child with Down syndrome increases in a gradual, linear fashion until about age 30 and increases exponentially thereafter (2). The risk of having a child with Down syndrome is 1/1300 for a 25-year old woman; at age 35, the risk increases to 1/365. At age 45, the risk of having a child increases to 1/30.

Prenatal SCREENING

Maternal serum screening :

Maternal serum screening can allow the detection of trisomy 21 pregnancies in women in younger than 35 years who give birth to about 70 percent of infants with Down syndrome. The serum markers most widely used are

• Alfa-fetoprotein
  
  
• Unconjugated estriol
  
  
• Human chorionic gonadotropin

This combination is known as "triple test"

The maternal serum levels of these proteins and of steroid hormones vary with the gestational age for the pregnancy. The triple test is usually performed at 15 to 18 weeks of gestation. The level of each serum marker is measured and reported as a multiple of the median for women with pregnancies of the same gestational age as that of the patient's. The likelihood of trisomy 21 is calculated on the basis of each of the serum marker results and the patient's age. A positive test is an indication for amniocentesis.

The triple test can detect 60 percent of trisomy 21 pregnancies; it has a false-positive rate of 5 percent. The likelihood of fetus having trisomy 21 in a patient with a positive test is about 2 percent. A normal result reduces the likelihood of trisomy 21 but does not exclude it. Test performance can be slightly improved by adjusting for maternal weight, ethnic group and insulin-dependent diabetes mellitus.

The triple test fails to detect 10 to 15 percent of trisomy 21 pregnancies in older age group. Therefore for women 35 years or older, maternal serum screening should not be offered as an equivalent alternative to amniocentesis or chorionic villus sampling.

Several demographic or pregnancy-related factors influence the biochemical levels. Taking these factors into account will enhance screening performance.

The factors are:

• Gestational age
  
  
• Maternal weight
  
  
• Maternal race


• Maternal insulin-dependant diabetes
  
  
• Multiple fetal pregnancy
  
  
• Family history of Down syndrome