This study aimed to set up a practical lab-side approach to discriminate fetal from maternal blood in samples obtained by cordocentesis. To determine the fetal origin of the blood, a modified Apt test was applied to 30 cases of prenatal diagnosis. A change of colour of the fetal and adult blood during the procedure was the hallmark to assess fetal origin. At the end of 60 s of the test, fetal blood yielded a pink colour whereas adult blood was dark green-brown. The test was repeated in mixtures of fetal and adult blood. The results suggest that the modified Apt test is a practical, quick, inexpensive, and efficient test to determine the origin of blood samples obtained by cordocentesis. However, it should be kept in mind that samples containing a mixture of both fetal and adult blood could also yield a fetal blood reaction. When maternal contamination is suspected, we propose that at least 30 metaphases from different slides should be counted. This could yield fetal as well as maternal chromosomes.