Reference range for cervical length throughout pregnancy: non-parametric LMS-based model applied to a large sample.

OBJECTIVE Short cervical length is an important risk factor for preterm delivery. However, because cervical length changes throughout pregnancy, adequate risk estimation needs to take into account the gestational age (GA) at which the measurement is taken. We aimed to model cervical changes throughout pregnancy in order to be able to use Z-scores, avoiding the confounding effect of GA. METHODS Cervical length was prospectively measured in singleton pregnancies, as part of routine antenatal care over a 3-year period. Measurements were taken at GA ranging from 16 to 36 weeks and only one measurement per pregnancy was used in the analysis. Because cervical length measurements are not normally distributed, we used a non-parametric approach (LMS method) to best describe the distribution of the measurements with gestation. RESULTS We included 6614 cervical length measurements. The LMS method identified changes in cervical length measurement across GA. We computed new reference charts and provide L, M and S values that allow the calculation of Z-score at any GA from any cervical length measurement 'Y' using the formula: Z-score = ((Y/M)(L) - 1)/(L x S). CONCLUSION Cervical length measurements do not have a normal distribution at a given GA and so require a statistical model that takes this into account. The model that we developed allows easy Z-score calculation, therefore avoiding the confounding effect of GA and allowing straightforward monitoring of cervical length.