Types of ultrasounds
The use of ultrasound
'Bonding' baby pictures and discovering the sex
The ultrasound procedure
Physical effects and research
Worrying concerns

Ultrasound tests (also known as 'sonograms' or 'scans') can be performed during pregnancy for a variety of reasons. Medical ultrasounds work on the same principle as sonar (used in oceanography to map the sea bed). The technician uses a hand-held ultrasound probe (or 'transducer') to generate and receive high frequency sound waves that cannot be heard by the human ear. Hundreds of sound waves are emitted from the transducer during each scan. These waves are absorbed and bounced back from human tissues, bones and body fluids (all with different densities) to create black and white ultrasound images that look similar to a photographic negative, with black areas indicating liquid mediums (such as amniotic fluid) and grey or white areas indicating denser materials such as tissues and bones.

The sound frequency of ultrasound is measured in megahertz (or MHz). Frequencies used for pregnancy ultrasounds can range from 1.6 to 10 MHz, but are more commonly between 3 and 7.5 MHz. Generally the lower the frequency, the further (or deeper) the sound waves can penetrate the body's tissues. Ultrasound waves that create images for visual examination are intermittently 'pulsed' to reduce the heating of the body's tissues (unlike continuous ultrasound therapies that may be used to treat injured muscles and tissues). 'Diagnostic ultrasounds' (that create images) tend to require lower intensities than Doppler ultrasound, used to assess blood flow through the cord and placenta and to listen to the baby's heartbeat. Ultrasound does not use radiation (like x-rays) and is seen by many caregivers as a non-invasive way to view the unborn baby, uterus and placenta during pregnancy. The physical effects and research into the safety of ultrasounds are looked at in