Almost all pregnant mothers in US hospitals get Ultrasounds, as it helps doctors monitor the progress of the fetus to insure everything is going well.
Some scientists have speculated that ultrasounds cause autism, which is potentially plausible because in the early formation of the human brain, it is beginning to make its connections.
Sound waves could disrupt this and cause a problem, but no one knows for sure, although some claim that they have evidence.
Recently, the Online Think Tank was contacted by a rather concerned person, who believes that the evidence shows it is not worth the risk to continue using the ultrasounds without warning, until we know more and stated during a long dialogue on the subject:

Yes, all these devices should be regulated and carefully controlled!...
I'm only asking for honesty...
open disclosure to all recipients, regarding the potential dangers of use, that's all!

So, some might content that perhaps your line of argument is; "Would you put your baby in a Microwave; then why are you bombarding your fetus with ultrasound?" Ultrasound is much different than microwave, but realize that ultrasound is sound frequency a low levels and it easily disrupt the vibrational energy at the cell level.
Probably not such a great idea for a vulnerable fetus in early stages of growth and there have been many studies that suggest there is a sinister correlation between ultrasounds and autism.
We need more research to know the truth.
The rates of Autism in the United States are 1:150 and that is way too high.