Recently Martin and colleagues10 compared SV parameters derived from EC and TTE in a sample of 44 healthy pregnant volunteers and found excellent agreement in mean left ventricular ejection time and heart rate. However, agreement in SV was poor with a percentage error of 42% in Bland-Altman analysis. Hypothesizing that both methodologies in estimating SV, EC and TTE, use measures for blood velocity and systolic ejection time, Martin et al. hypothesized that agreement between both methods could be significantly improved by calibrating SV-EC with the LVOT area that is derived from one time TTE measurement. Based on a multiplicative linear regression model and following log-linear transformation Martin et al. derived the following formula for calibration: