The most common method of biophysical fetal monitoring is recording and analyzing the cardiotocographic signals. In analysis of the fetal heart rate signal special emphasis is paid to the deceleration patterns and their correlation to the uterine contractions. According to deceleration classification the most important is the distinguishing between the periodic and the episodic types. In visual analysis, this classification is based on fuzzy description of deceleration onset being "abrupt" or "gradual". Application of commonly used interpretation of these imprecise terms in computer aided monitoring systems very often leads to erroneous classifications. Therefore, the redefinition of the deceleration nadir phase, as a group of samples around the lowest point, is required. It ensures that the onset phase, which is very important in deceleration classification, will consist of only appropriate samples. For determination of nadir the new method based on three stage-analysis of samples frequency distribution was developed. To evaluate the proposed method we compared the results with reference data obtained from clinical experts.