We determined the position of the conus medullaris (CM) relative to the vertebral column in premature and term babies and assessed its ascent in 99 children, using ultrasound. We related level to the middle of a vertebral body or an intervertebral space. Postmenstrual age at examination was related to the level of the CM with the Krus-kal-Wallis test. The children were divided into five age groups and the level of the CM was analysed with a chi-squared test. An increase in postmenstrual age was significantly associated with higher levels of the CM (P = 0.003). The CM ascended in the older children (P = 0.04). The CM is at the L2/L3 intervertebral space in premature babies of 27-29 weeks of postmenstrual age. In babies of more advanced gestation a statistically significant ascent of the CM is observed and the CM is at the adult level of the L1/L2 intervertebral space around the 40th postmenstrual week. No further ascent occurs thereafter.