A-mode units emit ultrasonic waves from a hand held transducer placed externally against the skin of the abdomen and directed towards the uterus. Ultrasound waves are converted to electrical energy in the form of audible or visual signal. The transducer is placed low in the right flank near the udder of the standing doe. Clipping a small area of hair in this region will allow optimal contact. A coupling agent such as commercial ultrasonic gel, K-Y jelly, carboxymethylcellulose lubricant or vegetable oil should be applied to the transducer to eliminate air spaced between the skin and the transducer head.
Some units emit a light or audible signal when a fluid-filled structure is detected.
Units with an oscilloscope display reflections as peaks or blips on the screen. Nonpregnancy is suggested when the peaks are present only in the left half of the screen. When a fluid filled structure is detected, peaks will also appear on the right half of the screen. The principle of Doppler ultrasound for pregnancy diagnosis is the detection of movements- blood flow in the middle uterine artery, umbilical arteries, fetal heart beat and fetal movements. Transducer emit ultrasound waves, sound reflected from motionless structures has the same frequency as the transmitted sound, whereas sound reflected from moving organ or blood has a different frequency.