OP20.02: Normal reference values for the anterior cerebral artery throughout pregnancy

Objectives: Monitoring growth restricted fetuses involves serial measurements of the middle cerebral (MCA) and umbilical artery (UA) pulsatility index (PI). The cerebroplacental ratio, i.e. MCA PI/UA PI, is a useful combination of the two indices. However, currently used reference ranges for the cerebroplacental ratio are based on cross-sectional data. For serial assessments reference ranges should be based on longitudinally derived data. Our aim was to establish longitudinal reference ranges and provide terms for calculating conditional mean and ranges for serial measurements in individual fetuses. Methods: 161 women with low-risk pregnancies were recruited to this prospective longitudinal study. The cerebroplacental ratio was determined using Doppler ultrasound. Each fetus was examined 3–5 times during the second half of pregnancy. Multilevel modeling was used in order to calculate means and percentiles, and terms for the conditional repeat measurement in individual fetuses. Results: A total of 550 sets of observations of the MCA and UA PI were included in the statistics. The cerebroplacental ratio changes with gestation during the second half of pregnancy in an inverted Ushaped fashion (Table). When compared with the non-conditional ranges, the conditional ranges are typically narrower and shifted towards the index measurement.