A longitudinal study of umbilical artery Doppler waveforms in normal pregnancy: analysis using Laplace transform techniques

Objective To analyse umbilical artery Doppler waveforms using the Laplace transform analysis technique, an alternative method of waveform shape analysis, and to determine the normal ranges for the variables of this technique throughout normal pregnancy. Design A prospective longitudinal study. Setting Department of Obstetrics and Gynaecology, Nottingham University Hospital. Sample A cohort of 17 healthy women investigated every four weeks from the end of the first trimester until term. Methods Umbilical artery Doppler signals were recorded and analysed using the Laplace transform analysis technique. The median and interquartile ranges for each variable were determined and serial changes during pregnancy described. Results Vessel wall tone decreases in the umbilical artery at the beginning of the second trimester. Alpha, the variable related to upstream flow conditions, also decreases at this stage of pregnancy but values are then comparatively stable from 24 weeks of gestation until term. A fall in downstream resistance within the fetoplacental circulation during pregnancy is detected using the C‐coefficient. Real pole appears to be of no value in the assessment of downstream resistance to flow in the fetus. Conclusions The normal ranges for the Laplace transform analysis variables have been established for the umbilical artery longitudinally through normal pregnancy. Changes in fetal blood flow during complicated pregnancy can now be investigated using this technique.