Long story short: excessive prenatal ultrasounds could be contributing to the worldwide increase in autism and developmental disorders. There are tons of scientific papers that discuss side effects of ultrasound that can cause brain damage. You can read more about it on the websites below.

The reason why doctors believe ultrasound is safe is because there is no evidence that it hurts people in epidemiology. However, it is impossible to perform valid epidemiological studies. Practitioners do not collect detailed information about scan sessions. So, no dose data is available to researchers.

I believe in evidence based care and First Do No Harm.

Sign a petition that aims to fix this:
http://www.change.org/petitions/health-risks-of-
prenatal-ultrasound-the-urgent-need-for-more-res earch-and-regulation

You can learn more about this issue from others.

Read the blog of internationally renowned baby-brainologist, Dr. MF Casanova -
http://corticalchauvinism.wordpress.com/

Read the independent research website of engineer Mrs. Parrish Hirasaki -
www.ultrasound-autism.org

Read the website of radiologist Dr. David A Toms (who does not believe ultrasound is harmful, but still is aware of these huge safety issues):
http://www.fetalultrasoundsafety.net/