Fractional spine length: a new parameter for assessing fetal growth

Sonographic measurement of a segment of the lower thoracic and lumbar fetal spine was performed on 218 normal fetuses of between 15 and 42 weeks' gestational age (GA) to establish a new parameter for assessing fetal growth. The increase of the fractional spine length (FSL) was represented by the equation: FSL = -37.62 + 4.36 (GA) - 0.04 (GA)2, with R2 = 0.92. The FSL also correlates strongly with fetal femur length (FL): FSL = 5.36 + 0.82 (FL), with R2 = 0.93. The ratio FSL/abdominal circumference (AC) X 100 was relatively constant at 20.4 +/- 3.1 (mean +/- 2 standard deviations) from 19 to 42 weeks in contrast to the ratio FL/AC X 100, which was constant at 22.3 +/- 2.4 (mean +/- 2 SD) only after 23 weeks. Nine of 10 small-for-gestational age fetuses examined during the study had a FSL within the normal range, as well as two fetuses who were thanatophoric dwarfs. The FSL provides a new ultrasonic longitudinal measurement of the fetus which may be used in addition to the femur length in fetal growth assessment.