Objective To determine the normal position of the fetal conus medullaris and assess its clinical significance.Methods A total of 550 singleton pregnant women who were examined by fetal ultrasonography in the Department of Ultrasound at the Second Hospital of Fujian Medical University from June 1, 2013 to September 31, 2014 were included.Inclusion criteria were pregnancies with regular menstruation cycle and known last menstrual period, gestational age (GA) confirmed by ultrasonographic examinations, and neonate abnormality excluded by pediatrician after born.Routine obstetric ultrasound examinations were performed to measure the biparietal diameter (BPD), femur length (FL), head circumference (HC) and abdominal circumference (AC).The conus distance (CD) was determined by measuring the distance between the distal end of the conus medullaris and the caudal end of the last vertebra.The average value was calculated for each group according to GA.Fifty normal fetuses were randomly selected for quality control.For inter-observer reliability assessment, the same data were collected and analyzed by two different operators.For intra-observer reliability assessment, the data were collected and analyzed twice by the same operator with an interval of half an hour.Linear regression correlation analysis was performed to analyze the relationship between CD and GA, BPD, FL, HC and AC.Intraclass correlation coefficient (ICC) and Bland-Altman analysis were used to analyze the repeatability of the analysis.Results In 518 (94.2％) of 550 fetuses, the CD was successfully measured.Positively correlations between the conus distance and the gestational age was observed.The CD was (10.0± 3.3) mm at ＞ 14-≤ 15 weeks (n=17), (27.7±3.8) mm at ＞ 20-≤ 21 weeks of gestation (n=18), (41.5±2.4) mm at ＞ 26-≤ 27 weeks (n=8), (54.7±3.0) mm at ＞ 32-≤ 33 weeks (n=17), and (71.9±2.7) mm at ＞ 41-≤ 42 weeks (n=6).Linear regression correlations between CD (mm) and FL, AC, HC, BPD and GA were:CD=1.04× FL (mm)-8.71, CD=0.23 × AC (mm)-10.11, CD=0.28× HC (mm)-18.10, CD=0.90× BPD (mm)-17.65, CD=2.34× GA (weeks)-20.94 (r=0.990, 0.985, 0.978, 0.974 and 0.973, respectively, all P ＜ 0.01).The measurement of the conus distance exhibited good reproducibility between different operators (ICC=0.984, 95％CI: 0.972-0.991) and for the same operator (ICC=0.992, 95％CI: 0.985-0.995) with the 95％ limits of agreement of-1.8 to 3.5 mm and-4.3 to 2.1 mm.Bland-Altman analysis showed that most of the measured values were within 95％ confidence interval suggesting good consistency.Conclusions The distance, between the end of the conus medullaris and the end of the last vertebral body under ultrasound scan, is positively correlated with FL and other ultrasonographic indicators, which implies that it could be used as a new index for position of fetal conus medullaris.