RESUMO
Objective:To assess the significance of counting the number of caudal vertebral ossification centers (OCN) below fetal terminal conus medullaris in the screening for closed spina bifida and tethered cord syndrome (TCS).Methods:The OCN was counted in 961 normal fetuses(normal group) between 17 and 41 gestational weeks and in 140 fetuses with closed spina bifida or tethered cord syndrome(abnormal group) from Jan.2013 to Dec.2020 in Affiliated Shenzhen Maternity & Child Healthcare Hospital, Southern Medical University, Women and Children′s Hospital, School of Medicine, Xiamen University and Maternity and Child Health Care of Guangxi Zhuang Autonomous Region. The OCN was counted in the dorsal mid-sagittal section of fetal caudal spine.The reliability and agreement test were evaluated by intraclass correlation coefficients in another 50 normal fetuses. The OCN was compared between two groups. ROC curve and the cut-off value were constructed and calculated.Results:In normal group, the N increased with the growing of gestational age.In the subgroup of 17-20 weeks, the OCN ranged from 5 to 7 in most fetuses. In the others subgroups, the OCN was equal to or greater than 6 in 99.9% cases and more than 6 in 97.1% cases. In abnormal group, OCN was less than 7 in 93.0% fetuses and less than 6 in 82.8% cases. There were statistical differences between the two groups except for the subgroup of 17-20 gestational weeks( P<0.05). With the cut-off value of 6.5, the specificity and sensitivity were 93.0% and 94.3% respectively for predicting the presence of closed spinal dysraphism or TCS. Conclusions:OCN is a simple way to evaluate the position of conus medullaris and to screen for the skin-covered spine dysraphism or TSC. OCN is more than 6 in most normal fetuses. Further evaluation of spine is required in fetuses with N less than or equal to 6.