Background: An abnormally high [above mean + 2 standard deviations (SD)] umbilical artery (UA) pulsatility index (PI) indicates impaired fetal outcome, whereas the impact of an "abnormally" low (below mean -2 SD) PI is unknown.
Methods: Perinatal outcome was compared between cases with a UA PI less than mean -2 SD (group A: high-risk cases selected from a database, n = 330; group B: unselected cases, n = 39) and unselected controls (group C) with a PI within mean +/- 2 SD (n = 863) at Doppler velocimetry. Groups B and C were retrieved from a population-based sample. The unpaired t-test, Mann-Whitney U-test, chi(2)-test and Fisher's exact probability test were used for statistical comparisons with a two-tailed p < 0.05 being significant.
Results: No significant differences were found between group A vs. group C and group B vs. group C regarding perinatal mortality, Apgar scores at 1, 5 or 10 min, or arterial or venous cord blood pH. Postterm pregnancy in group A carried no additional risk. For obvious reasons, operative delivery and neonatal intensive care were more common in group A than in group C, but no such differences were found between groups B and C. The mean birthweight was 3.7% higher in group B than in group C (p = 0.049).
Conclusions: Deeming a UA PI below the lower reference limit as "abnormally" low is a statistical definition that was not reflected by a biological imperfection. Instead, a low UA PI promoted fetal growth.