Chromosomal Defects are one of the commonest congenital Defects responsible for fetal and neonatal death and handicap. The traditional method of screening for Down’s syndrome has been maternal age where amniocentesis or chorionic villous sampling is offered to women aged 35 years or more. This results in the need for an invasive test in 15-20% of pregnant women with a detection of less than half of the fetuses with Down’s syndrome, because the majority of affected fetuses come from the younger age group. A more effective method of screening is based in the combination of: maternal age, a maternal blood sample for the measurement of the placental products of free b-hCG and PAPP-A and an ultrasound scan at 11-13 weeks; to measure the collection of fluid behind the fetal neck (nuchal translucency), to examine the fetal nose and palate, to measure the fetal heart rate and to assess the flow of blood across the tricuspid valve of the fetal heart and the ductus venosus.This new method of screening reduces dramatically the number of women requiring an invasive test from about 20% to less than 3% and at the same time increases the detection rate of Down’s syndrome and other major chromosomal abnormalities from less than 50% to more than 95%.There are several other benefits of the 11–13 weeks scan as well including: accurate dating of the pregnancy, early diagnosis of many major fetal abnormalities, and the detection of multiple pregnancies with reliable diagnosis of chorionicity, which is the main determinant of the outcome in multiple pregnancies Although there are some markers at second trimester (sonographic and biochemical) to assess risk of chromosomal abnormalities but detection rate is less than first trimester and also early diagnosis ( in first trimester) and early termination is much better regarding patient’s preferences, less complications and less psychological stress.