Objectives: To determine the position of the conus medullaris during pregnancy in relation to the last vertebral body and to examine its use to detect skin covered, closed spinal bifida. Methods: Retrospective study involving 300 consecutive ultrasound examinations between 16 weeks of gestation and term. Two operators independently assessed the images of the spine on whether the conus medullaris and the last vertebral body were visualised in one image in a midsagittal plane. The distance (conus distance/CD) between these two landmarks was measured twice by both operators, who were not aware of any previous measurements. Intraand interobserver variability was assessed by 95% limits of agreement. Linear regression analysis was used to determine the relevant contributors to the CD and a normal range was computed based on the best-fit model. The normal results were compared with five cases of prenatally detected skin covered spinal dysraphism. Results: In 84.7% of the 300 cases both operators were able to visualize the conus medullaris and the last vertebral body. 95% limits of agreement for the intraobserver variability were ±1.9 mm. For the interobserver variability they were −3.7 and 2.5 mm. We found a linear relationship of CD to gestational age, biparietal diameter and abdominal circumference. The strongest relationship was observed for femur length (CD = −8.2+ × femur length). In the five abnormal cases, CD was well below the 5th percentile. Conclusions: The determination of CD allows for an objective and feasible assessment of the conus medullaris position. The parameter promises to be very useful for the prenatal detection of skin covered spinal dysraphism.