There are two types of sonography for gynaecology and pregnancy evaluation. One of these is done from the skin surface and is called a trans-abdominal scan. The other is done with the probe inside the vagina and this is known as a transvaginal scan. These have been in use for many years now and have been critically and scientifically evaluated by many workers and are proven to be safe beyond doubt.
With transvaginal scans the probe is adjacent to the organ of interest and a very clear picture can be obtained because the frequency of the transmitted wave can be higher, thereby resulting in a higher resolution. With trans-abdominal scans, the beam has to penetrate a longer depth to obtain information. This degrades the resolution. The higher the frequency, the better the resolution but the lesser the depth the beam can penetrate. A combination of the technique may be necessary in some patients.