Purpose: An abnormally high (above 95th percentile) pulsatility index (PI) in the umbilical artery (UA) indicates impaired fetal outcome, whereas the impact of an ''abnormally'' low (below 5th percentile) PI in the second trimester is unknown.
Materials and methods: Between 2001 and 2004, second trimester pregnancy screening was performed at our institution on 5220 patients, and additional UA Doppler ultrasound on 547 (10.5%) of these patients, between 16th and 25th week of pregnancy. Files were available for analysis in our electronic database (PIA Fetal Database (GE, USA)). To evaluate the association of a UA PI below the 5th percentile in the second trimester of pregnancy with fetal anomalies and outcome, a retrospective database analysis was conducted.
Results: In 25 fetuses (3.6%), a UA PI below the 5th percentile was found. In 7 of these fetuses (28%), abnormalities of the urogenital tract were diagnosed. There were multiple pregnancies in 7 cases (28%), consisting of two triplets, each including one fetus with malformation, as well as five monochorial twin pregnancies, including 2 fetuses with malformations. In 11 pregnancies (44%), second trimester screening showed no anomalies.
Conclusion: Only in 11 cases (44%) of fetuses with UA PI below 5th percentile in the second trimester, course and outcome of pregnancy were favourable, if defined as live birth without fetal malformations and uneventful neonatal period. The aetiology of an abnormally low UA PI in the second trimester seems to be diverse. Possible aetiological mechanisms will be discussed.