Pregnancy health

Here's how ultrasounds actually work

Plus, the one type of ultrasound that doctors say you shouldn’t get

Here's how ultrasounds actually work

Photo: iStockphoto

During a healthy pregnancy, going for an ultrasound can be a pretty thrilling experience. It’s a chance to see your baby’s profile, wee little feet and, if you desire, whether you’re carrying a boy or a girl. Some parents, however, wonder if all that checking in on babe is safe.

The good news is that ultrasounds, which have been around for 30 years, have a good track record for safety. “Studies continue to make sure ultrasound is safe. There is no reason to think that it harms mothers or babies,” according to the Society of Obstetricians and Gynaecologists of Canada.

Do ultrasounds use radiation?

Ultrasounds—sometimes referred to as sonograms, although that's technically the name of the picture produced—don’t use radiation but rather they use sound waves that are too high-pitched to be heard by the human ear. The technician holds a scanning device called a transducer, which focuses these waves on different parts of your belly. The sound waves bounce back from organs inside your body, and the transducer picks up the sound waves, which then form an image that can be seen on the monitor. (Hello, baby!)

Ken Lim, the division head of Maternal Fetal Medicine in Obstetrics and Gynaecology at BC Women’s Hospital, says there’s no evidence that the baby can feel or hear the sound waves as the frequency is above the audible range.

How many ultrasounds are safe?

The SOGC says all women should have an ultrasound during pregnancy, and the best time to do so is between 18 and 22 weeks. Here, the sonographer is checking the levels of the amniotic fluid around the baby, the position of the placenta and the vessels in the umbilical cord, as well as measuring various parts of your baby to assess the development of baby’s brain, heart and other major organs, spine, and face. Your doctor or midwife may also order ultrasounds earlier or later in the pregnancy to check on the same things. It’s also fairly common to have an ultrasound in the first trimester between 11 and 14 weeks to help establish your due date. Other reasons for ultrasounds at different stages of the pregnancy include checking on the placenta and the amount of amniotic fluid around the baby, seeing what position the baby is in, and to check on the baby’s growth.


“We only order them for medical reasons,” says Lim. “We go by the ‘ALARA’ principle, which is ‘as low as reasonably achievable.’ In other words, we try to minimize the energy outputs to the point where we use just enough to see what we need to do.” That’s partly because of the theoretical side effects from ultrasounds that may include a slight heating or agitation of tissues.

However, says Lim, “It takes a fair amount of energy output over a long period of time in order to get that effect, the machine is doing pulse, listen, pulse, listen. The other thing is that energy has to be staying in the same place to produce an effect, but we’re always moving the transducer around. It’s not like we’re focussing on something for 5 or 10 minutes at the same spot. The threshold levels for safety are pretty high.”

That said, you’re entirely within your rights to ask your doctor or midwife if the ultrasound they are sending you for is necessary.

 Are 3D ultrasounds safe?

Some parents wonder if 3D ultrasounds or 4D ultrasounds (which show movement) are stronger than a standard 2D ultrasound, but Lim says that if you’re at a regulated centre at a hospital or medical clinic, all machines must adhere to safe levels of heat and mechanical effects that have been established by Health Canada and other agencies.


That said, 3D “keepsake” or “entertainment” ultrasounds done for non-medical reasons by a commercial business are not recommended by Health Canada, the SOGC, Sonography Canada, the Canadian Association of Radiologists and the US Food & Drug Administration.

“Non-medical ultrasound centres are unregulated and not accredited by any professional body that we are aware of,” says Tara Chegwin, professional services coordinator for Sonography Canada. “There is no professional standard around them. They may not have any patient safety controls, they may not have any technical safeguards and we don’t know the level of operator expertise and governance over training.” The FDA notes one of the biggest concerns with keepsake ultrasound centres: there's no limit on how long the imaging session will last. In some cases you may be there for up to an hour to get 'good' pictures of your babe, or be asked to come back for multiple sessions.

So it’s totally understandable to want every chance to have a peek at your baby, but it’s best to understand why your health care provider is recommending an ultrasound, and to stick to accredited ultrasound centres. Pretty soon you’ll be able to bond with your babe, and share the experience with friends and family, as much as you like.

Read more: What to expect during ultrasounds Which over-the-counter medications are safe during pregnancy?

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Bonnie is a copywriter, editor and content consultant based in Thunder Bay, Ontario. She is also the founder and principal at North Star Writing. More of her work can be found in publications like Canadian Living, Best Health, and Chatelaine