Ultrasonography is a mechanism using high frequency waves to identify different structures in the body. The high frequency waves bounce off the tissues. The scans they produce depend on the different density of the tissues of the body.
X-rays are different to ultrasound pictures. Whereas fluids can be difficult to see with x-rays they can be easily diagnosed with ultrasound. Also ultrasound is a moving picture and x-rays are static. This enables us to see hearts beating in pregnant animals.
Ultrasound can also be performed without a general anaesthetic if the patient is co-operative but it usually entails some shaving as the fur can interfere with the ultrasound pictures. We would usually use ultrasound in conjunction with other diagnostic procedures to decide what is wrong with our patients.
We would usually recommend a pregnancy ultrasound for your pet if they are 4 weeks pregnant or more. Ultrasounds cannot identify how many there will be in a litter, so an x-ray at 6 weeks gestation may be helpful in determining if it is a small or large litter. Sometimes there can be birthing difficulties if there is only one puppy or kitten, due to their larger than normal size.