The Effect of the Pressure Exerted on the Maternal Abdominal Wall by the US Probe on Fetal MCA Peak Systolic Velocity.

To quantify the pressure exerted on the maternal abdominal wall during ultrasound examination and evaluate its effect on the fetal middle cerebral artery (MCA) peak systolic velocity (PSV).  Gravid women with singleton pregnancies in their 2nd-3 rd trimester undergoing fetal sonographic evaluation for various indications were recruited. Each subject underwent transabdominal US measuring fetal distance from the probe, abdominal thickness, amniotic fluid index and biophysical profile. The applied pressure was measured simultaneously using an electronic pressure sensor attached directly to the US probe. For each subject baseline values of the pressure required for proper visualization were obtained. Fetal MCA was then demonstrated using color Doppler US. The PSV was measured at different pressure ranges with each subject used as her own control. Care was taken not to exceed the baseline pressure for each subject.  29 women were recruited. 24 subjects (82.7 %) demonstrated a statistically significant positive correlation between the pressure exerted and MCA-PSV (R-0.37, p < 0.0001). Of these, 4 subjects (13.8 % of study population) demonstrated elevation of PSV values above 1.29 MOM and 5 subjects (17.2 %) demonstrated elevation of PSV values above 1.5 MOM for gestational age with increasing pressure. In total, 9 subjects (31 %) demonstrated significant changes in the MCA-PSV measurements (owing to increase in pressure applied) that could potentially falsely influence clinical obstetric diagnosis and management.  The pressure exerted on the maternal abdominal wall during US examination is an important parameter, producing clinically significant measurable changes in fetal MCA hemodynamics. Further study is needed in order to demonstrate the potential effect of pressure as a parameter influencing the diagnostic accuracy of the MCA-PSV in the setting of fetal anemia.