Human beings possess 23 pairs of chromosomes. 22 of these are true pairs of similar chromosomes, the 23rd consists of either two X chromosomes, in females, or and X and a Y chromosome, in males.

Trisomy is a condition in which an individual deviates from normal pattern by possessing three copies of particular chromosome. Aneuploidy is a more general term for disorders involving the wrong number of chromosomes.

Prenatal testing for trisomies such as Down syndrome, carried out in the first trimester of pregnancy has the major advantage of providing more time for consideration and action if the risk of anomaly is found to be high. The screening might take place in the first trimester, second trimester or both trimesters and might involve from one to six markers in addition to maternal age. Summary of the behavior of markers associated with fetal defects in the First Trimester:

Marker Down Syndrome (T21) Edward Syndrome (T18) Patau Syndrome (T13)
AFP Lower- 25% Lower- 35-55% Slightly raised
uE3 Lower- 30% Lower- 35-55% Lower- 30%
Free HCG? Raised - 2X Lower- 50% Lower- 50%
PAPP-A Lower- 50% Lower- 50% Lower -50%
Inhibin-A Raised - 2X No value Raised - 2X

The prenatal screening combines First Trimester Screening (FTS) that includes Nuchal Translucensy (NT) and free HCG? and PAPP-A at 11-13 weeks, accuracy level of 87% and Triple test in Second Trimester includes AFP, free HCG or HCG? and uE3 (Estriol) at 14-20 weeks, 60-65% accurate. The overall Integrated Test (IPS) is 93% which is quite high detection rate, it involves holding back information obtained at the first trimester that could possibly allow earlier intervention.

A maternal serum marker suitable for trisomy screening has the following characteristics:

  • It is associated with a clearly elevated or reduced concentration in maternal serum when the fetus is affected by an anomaly
  • It can be measured accurately, on a large scale and at reasonable cost