Ultrasonography relies on the use of high-frequency sound waves beyond the limit of human hearing to image internal body structures. Unlike diagnostic radiology, obstetric ultrasound does not use ionizing radiation, making it safe for developing fetuses. A pulse of sound is sent into the body, and the machine listens for the echoes that bounce back from different tissue layers. By timing how long the echo takes to return and measuring its strength, the machine constructs a real-time grayscale image on a monitor.
Sound waves require a physical medium such as soft tissue or fluid to travel through; they cannot propagate in a vacuum. Key wave parameters include frequency, which is the number of wave cycles per second measured in hertz, and wavelength, the physical distance covered by one complete cycle. In diagnostic imaging, frequency and resolution have an inverse relationship with tissue penetration. Higher frequencies provide superior image detail but poor depth penetration, which suits superficial structures. Lower frequencies penetrate deeper into the pelvis or abdomen but yield lower image resolution.
The transducer acts as the mouth and ear of the ultrasound machine, converting electrical energy into sound and sound back into electricity. This capability relies on the piezoelectric effect found inside special crystals lining the probe face.
Acoustic impedance measures how much resistance a tissue gives to the passage of sound waves. It depends on the density of the tissue and the speed at which sound travels through it. When a sound beam hits the boundary between two tissues with very different acoustic impedances, a large portion of the sound bounces back as an echo, while the rest continues forward. Bone and air have extreme impedances compared to soft tissue, creating strong reflections or complete barriers that block deeper imaging.
As ultrasound waves travel through the body, their intensity decreases through a process called attenuation, which is caused by absorption, scattering, and divergence. Some waves bounce straight back to the probe (reflection), allowing us to see organ borders. Other waves bend as they pass through angled tissue boundaries (refraction), which can sometimes create image artifacts. Midwives must understand that deep pelvic structures require adequate gain adjustment to compensate for the signal loss caused by attenuation.
Ultrasound data can be displayed on a screen in several different formats depending on clinical needs.
Ultrasonography is an invaluable tool in midwifery for confirming intrauterine pregnancy, estimating gestational age, checking placental location, and monitoring fetal well-being. Although ultrasound uses non-ionizing mechanical waves, safety guidelines still apply. Midwives should follow the ALARA principle—As Low As Reasonably Achievable—ensuring acoustic power output and exposure time are kept to the minimum necessary for diagnostic clarity, particularly during early embryonic development.