Obstetricians often order ultrasounds as early as eight weeks gestation. Many mothers have concerns about the sound waves possibly interrupting the baby's growth and development process that early in the pregnancy.
Ultrasounds during the first trimester of pregnancy seem to be safe and in many cases, crucial to confirm whether a pregnancy is normal or if there are potential problems. One of the most accurate screening tests available for Down's syndrome involves an ultrasound in the first trimester. Ultrasounds are also used to access the foetal position in the womb. Sometimes the egg will implant in a fallopian tube resulting in an ectopic pregnancy, which can be a life threatening condition for the mother.
Having said that, testing does involve an energy which can theoretically lead to temperature increases in the womb if the foetus is exposed to the ultrasound sound waves for an extended period of time. Some types of ultrasound tests do expose the foetus to more energy (longer periods of time) and therefore a higher risk of temperature increases than other tests. The point to be made here is that ultrasound testing can be an important diagnostic tool during pregnancy and should always be done by an experienced technician or obstetrician.