A method for rapid estimation of fetal blood content during cordocentesis is described. This procedure gives an opportunity to determine the contamination of the fetal sample by maternal blood as soon as possible. The method is based on the ability of fetal hemoglobin to resist denaturation in alkaline conditions, and can be used routinely. Fetal blood samples show a lower degradation rate (range 1.2-8.2%) compared to the maternal samples (range 25.0-52.5%). The method has the capacity to discriminate fetal and maternal samples with regard to their fetal hemoglobin content.