Fig. 1. Mouse fetal ultrasound imaging. (A) In utero 2D-ultrasound image of a fetus, showing two white arrows used to measure the crown to rump length. (B) A fetus showing pericardial effusion (red arrow) and hydrops (yellow arrows). (C,D) Color flow Doppler analysis showed outflow regurgitation in an E18.5 fetus. Aliasing (see arrow) associated with the outflow (C) indicated increased velocity. Superimposed on the outflow is a regurgitant diastolic flow (D). (E) Spectral Doppler analysis revealed an abnormal regurgitant flow. (F) M-mode images from an E17.5 fetus, obtained from a short axis view (see diagram), show the position of the right (RV) and left (LV) ventricular walls and the interventricular septum (IVS) through multiple cardiac cycles. Wall thickness, and chamber volume in diastole and systole can be obtained by measuring the distances between numbered positions (red color dots versus corresponding position in M-mode image).