Down’s syndrome is a chromosomal disorder, so screening for Down’s syndrome is an essential prenatal test for every woman who is pregnant. Down’s syndrome screening is divided into early screening and mid-term screening. Both types of screening are done for the same purpose, but there are a few differences. First of all, the timing of early and mid-term Down’s syndrome screening is different: early Down’s syndrome screening is performed between 11-14 weeks of pregnancy and mid-term Down’s syndrome screening is performed between 16-18 weeks of pregnancy. Secondly, there is a difference in the way the screening is performed between the two periods. Early screening is based on the ultrasound results of the posterior nuchal translucency (NT) screening and the results of the serum biochemical tests to assess the risk of Down’s syndrome in the fetus. Midtrimester Down’s screening is a more common test that involves taking maternal serum and performing tests for alpha-fetoprotein, chorionic gonadotropin concentration, free estriol and inhibin. The risk factor for a Down’s syndrome baby is calculated based on the elevation or decrease of these markers in the maternal serum, combined with the pregnant woman’s due date, age, weight and gestational week at the time of blood collection. The results of early and mid-term Down’s syndrome screening are more accurate than those of early Down’s syndrome screening. The results of early Down’s syndrome screening are affected by a variety of factors, such as ultrasound, the doctor’s technique and the position of the fetus. The results of Down’s syndrome screening in the second trimester are less affected by external factors, so the accuracy of the results is higher than in the early trimester. The screening test is only a risk assessment of the fetus for Down syndrome, but not a confirmatory test, and further amniocentesis or non-invasive DNA is needed to confirm the diagnosis if the fetus is found to be at high risk.