As the word ultrasound means ultra+sound these are just sound waves > 20000 HTZ. The normal range of sound which we hear is 20-20000 hertz. Sound waves greater than 20000 Hz which are beyond our hearing capacity is an ultrasound which can be heard by bats only. When soundwaves reflect from a surface they produce echos which inturn produces images.
Ultrasound is totally safe in pregnancy and has got no side effects on the baby at any time. it is being used since 35 yrs for doing pregnancy scans and no evidence suggest that it is not safe . Ultrasound is done at different stages of pregnancy which should be done at that particular time to increase the efficiency of results . We do multiple ultrasound because different problems develop with growth of baby and ultrasounds have to be done at that particular time to detect the problem.
The various ultrasounds done in pregnancy are:
The first pregnancy ultrasound is done to locate the position of the baby which can also detect the heartbeat and this is called viability scan . Sometimes a repeat scan may be required if the heart beat is not developed and which could be developed at a later stage.
The 2nd ultrasound is done at 11-13 weeks which is the earliest scan to detect fetal abnormalities and is called NT/NB scan .
The 3rd ultrasound is done at 18-20 weeks for congenital anomalies of the baby and is also known as TIFFA or level II scan .
The 4th ultrasound is optional and is a dedicated scan for fetal heart and is done at 22-24 weeks and is called fetal ECHO.
The 5th ultrasound is done any time between 30-34 weeks depending on the growth of the fetus to detect the blood flow to the baby it also tells us the growth of the baby .Growth occurs maximum during this period and it is crucial to detect the growth lag (if any) and the cause of the lag .
The 6th ultrasound is usually done to check the descent ( coming down of baby ) cord around the neck and expected delivery time. This could be a optional scan
As ultrasound images formation depend on reflection or bouncing back of echos if ultrasound is used for prolonged periods it could raise the temperature is womb and if temperature is raised more than 40c it could cause harm to baby . However with the newer machines no or minimal heating effect is produced and even after continuously scanning for 30 minutes. After 30minutes the temperature might rise by 10c which is insignificant. .
Hence ultrasound done should be limited to small durations however can be repeated multiple time as required and there is no harm produce to baby by repeated ultrasound .