AbstractCell Fee DNA (cf DNA) in maternal circulation is derived from both mother and fetus.Prenatal screening for aneuploidies can be performed using these cfDNA in maternal blood. It is new noninvasive technique which provides highly accurate results but it is still screening test due to very infrequent false positive& false negative results. CfDNA is highly fragmented hemopoitic cells. Each fragment is between 50 to 200 base pairs. There is clear pattern to fragmentation size and patterns differ between maternal &fetalcfDNA. Maternal clearance of fetalcfDNA occurs in 2 days after delivery. Half life of cfDNA is approximately one hour.