In a hospital I can see a long queue of pregnant women waiting for their turn for ultrasound. Is it safe to go through the ultrasound during pregnancy, especially during last few weeks? Is ultrasound really required? What if it is not done? How does ultrasound affect baby?
As Luke points out, the ultrasound is very safe. However, in many cases it is not required. Typically by 20 weeks into the pregnancy, and sometimes earlier, a baby's sex can be determined by examining the genitals via ultrasound. My experience is that this is the most common reason for an ultrasound during pregnancy.
Of course if there are complications during the pregnancy, an ultrasound might be helpful to visualize the fetus and diagnose any problems. However there is no definite need for an ultrasound during the course of a normal pregnancy.
The U.S. Food and Drug Administration (FDA) has the following information regarding the risks of ultrasound imaging;
Ultrasound imaging has been used for over 20 years and has an excellent safety record. It is non-ionizing radiation, so it does not have the same risks as x-rays or other types of ionizing radiation.
Even though there are no known risks of ultrasound imaging, it can produce effects on the body. When ultrasound enters the body, it heats the tissues slightly. In some cases, it can also produce small pockets of gas in body fluids or tissues (cavitation). The long-term effects of tissue heating and cavitation are not known.
Because of the particular concern for fetal exposures, national and international organizations have advocated prudent use of ultrasound imaging. Furthermore, the use of diagnostic ultrasound for non-medical purposes such as fetal keepsake videos has been discouraged.
I had a bit of a look through PubMed and Scopus for any papers relating to ultrasound risk, but didn't find any.