In certain situations, testing the fetal blood directly is required. Commonly, when there is a fetal condition called hydrops that results in accumulation of fluid inside the fetus, fetal blood sampling is required to find out the cause. In other situations, the reasons for testing are essentially the same as that of chorionic villus sampling or amniocentesis. Generally, when the testing has to be done beyond 20 weeks, FBS is preferred over amniocentesis.