To evaluate the position of the fetal conus medullaris during pregnancy and its value in detecting tethered cord syndrome(TCS).

Nine hundred and seventy-four normal fetuses and 46 fetuses with TCS between 15 and 41 weeks gestation were involved in the study. Parameters D1 (the distance between the end of the conus medullaris and the caudal edge of last vertebral body ossification center) and D2 (the distance from the end of the conus medullaris to the caudal skin namely the intersection point of the extending line of D1 and the skin) were measured in the caudal midsagittal plane of the spine. Sixty normal fetuses were chosed randomly for interobserver variability. Correlation analysis between these two parameters and gestational age(GA) were conducted and the normal reference value of these parameters were calculated in normal group. The ratios of growth parameters (Biparietal diameter, Head circumference, Abdominal circumference, Femur length) to D1 and D2 were calculated separately to observe the difference of the ratios between two groups. All the parameters and ratios of normal fetuses were compared with that of TCS cases.

There was no significant difference in D1 and D2 between two observers. A significant linear correlation between the parameters and GA was found in normal group, linear regression equations were D1=0.251 GA-2.265 cm (R²=0.926, P<0.01) and D2=0.267 GA-1.812 cm(R²=0.928, P<0.01), respectively. D1 and D2 were much lower in normal group than in abnormal group (all P<0.01). The ratios of the growth parameters to D1 and D2 were relatively stable and had statistically differences between two groups in different gestational age.

The methods that determination of D1 and D2 are simple and feasible, and could help to the prenatal diagnosis of TCS.

Ultrasonography; Prenatal; Fetus; Conus medullaris; Tethered cord