A method for determining the fraction of cell-free fetal nucleic acids in a peripheral blood sample from a pregnant woman, comprising: (i) performing sequencing on cell-free nucleic acids contained in the sample so as to obtain a sequencing result consisting of a plurality of sequencing data; (ii) determining the number of the cell-free nucleic acids of a length falling into a predetermined range in the sample based on the sequencing result; and (iii) determining the fraction of cell-free fetal nucleic acids in the sample based on the number of cell-free nucleic acids of a length falling into the predetermined range, wherein said predetermined range is determined prior to step (i) by the following steps: (a) determining lengths of the cell-free nucleic acids in a plurality of control peripheral blood samples from pregnant women in each of which the fraction of cell-free fetal nucleic acids is known; (b) setting a plurality of candidate length ranges and determining the percentage of cell-free nucleic acids obtained from each of the plurality of control samples present in each candidate length range; (c) determining a correlation coefficient for each candidate length range based on the percentage of cell-free nucleic acids obtained from each of the plurality of control samples present in each candidate length range and the known fraction of cell-free fetal nucleic acids in each of the control samples; and (d) determining at least one candidate length range or a combination of the candidate length ranges as the predetermined range based on the maximum correlation coefficient. (i) performing sequencing on cell-free nucleic acids contained in the sample so as to obtain a sequencing result consisting of a plurality of sequencing data; (ii) determining the number of the cell-free nucleic acids of a length falling into a predetermined range in the sample based on the sequencing result; and (iii) determining the fraction of cell-free fetal nucleic acids in the sample based on the number of cell-free nucleic acids of a length falling into the predetermined range, wherein said predetermined range is determined prior to step (i) by the following steps: (a) determining lengths of the cell-free nucleic acids in a plurality of control peripheral blood samples from pregnant women in each of which the fraction of cell-free fetal nucleic acids is known; (b) setting a plurality of candidate length ranges and determining the percentage of cell-free nucleic acids obtained from each of the plurality of control samples present in each candidate length range; (c) determining a correlation coefficient for each candidate length range based on the percentage of cell-free nucleic acids obtained from each of the plurality of control samples present in each candidate length range and the known fraction of cell-free fetal nucleic acids in each of the control samples; and (d) determining at least one candidate length range or a combination of the candidate length ranges as the predetermined range based on the maximum correlation coefficient. (a) determining lengths of the cell-free nucleic acids in a plurality of control peripheral blood samples from pregnant women in each of which the fraction of cell-free fetal nucleic acids is known; (b) setting a plurality of candidate length ranges and determining the percentage of cell-free nucleic acids obtained from each of the plurality of control samples present in each candidate length range; (c) determining a correlation coefficient for each candidate length range based on the percentage of cell-free nucleic acids obtained from each of the plurality of control samples present in each candidate length range and the known fraction of cell-free fetal nucleic acids in each of the control samples; and (d) determining at least one candidate length range or a combination of the candidate length ranges as the predetermined range based on the maximum correlation coefficient.