Image resolution can be axial, lateral or temporal. Temporal resolution (or frame rate) is dependent upon your imaging area and the depth to which you are scanning, and is not frequency-dependent.

Lateral resolution (horizontally across the image) has some frequency dependence (beam width varies with depth and is due primarily to aperture size and frequency, with higher frequencies producing a narrower beam width), but the most direct link between frequency and resolution is in the axial direction.

Axial resolution is your ultrasound machine’s strength, and any measurements taken in this direction will be more accurate than those taken in the lateral direction. Axial resolution is dependent upon the length of your ultrasound pulse (it is roughly half the spatial pulse length), and given that lower frequency sound waves are longer than higher frequency ones, it can be appreciated that lower frequency transducers will have longer pulse lengths – and thus poorer axial resolution.

Of course, axial resolution also depends upon the number of cycles in the pulse, and higher quality machines will also have fewer cycles. This is due to them employing higher quality materials (such as the transducer backing layer and matching layer) in probe construction, to eliminate unnecessary 'ringing' without compromising sensitivity and efficiency.

For pregnancy scanning, axial resolution can have a big impact not only upon the appearance of the image, but on measurement accuracy and calculations of gestational age. Studies have also shown that the clarity of image produced by higher frequency transducers also results in greater reproducability, in other words, you will find that there is less varability in the results of your measurements when performed at higher frequencies. As a general rule, always start high, reducing your frequency only if you fail to achieve adequate penetration.