What to expect during ultrasounds

Pregnant mom Karin Field of Toronto had two questions she hoped an ultrasound would answer. One, would her expected third baby be healthy? And two—since her previous two babies had been born a few weeks early—was her cervix beginning to shorten up too early?

While an ultrasound is an opportunity to get a first look at your baby, it can also answer questions about your expected baby and about the baby’s environment—the location of the placenta, amount of amniotic fluid and condition of the cervix, for example.

By measuring the baby’s size, the ultrasound can also be used to give a good estimate of the baby’s age and expected date of birth. It can tell you if there is more than one baby and can detect some problems in the baby’s development and signs of genetic problems. One part of many ultrasounds is the measurement of the nuchal translucency—a fold on the back of the baby’s neck—which may indicate a higher risk that the baby has Down’s syndrome.

Depending on the baby’s position—some modestly keep their legs crossed during the procedure—the ultrasound might reveal the baby’s sex.

If you’re unsure about what happens during an ultrasound, we’ve compiled some basic facts before you head into the examining room.

What is an ultrasound?

The ultrasound machine uses a transmitter (held in the technician’s hand) to send high-frequency sound waves into the mother’s uterus. Although they are sound waves, they are so high in frequency that humans can’t hear them. When the waves hit the baby’s bones and other tissues, they are bounced back and picked up again by the transmitter, then translated by the computer into images. This procedure has been widely used for over 35 years and is considered to be very safe.

When are ultrasounds done?

An ultrasound can be done at any stage of pregnancy. One might be recommended early in your pregnancy if you’ve had some bleeding or if the doctor suspects the embryo has implanted in the wrong place (as in an ectopic pregnancy).

Many caregivers routinely recommend an ultrasound at around 18 or 19 weeks to check for Down’s syndrome and other abnormalities, as well as confirming the baby’s due date.

Later in pregnancy, an ultrasound can be done to determine the cause of unexpected bleeding, to find out the baby’s position, to estimate the baby’s size and learn other information. In some cases, such as when the baby’s growth rate needs to be monitored, a series of repeated ultrasounds may be recommended.

How do you prepare for an ultrasound?

Karin Field, who had an ultrasound scan at 18 weeks gestation, says she had no special instructions to prepare for the test. “I didn’t have to drink water or change into a hospital gown or anything; just pull my shirt up and my pants down as I lay on the table. The technician put a towel over the waistband of my pants to protect it from the gel,” she says.

In some locations, and depending on the stage of pregnancy, you may be asked to drink several glasses of water before the scan is done so that your full bladder pushes up your uterus and makes it easier to get a good picture of the baby. Your doctor or midwife will give you the instructions when he or she gives you the referral form.

Darcie Light of Richmond, BC, had a trans-vaginal first-trimester ultrasound, and says: “Preparing for this involved emptying my bladder, undressing from the waist down, covering myself with a sheet, and getting positioned on the exam table with my feet in the stirrups.” The technician used a lubricated probe inserted into the vagina.

“The technician asked me to insert the probe myself, then moved it around to get the images needed,” she says. This trans-vaginal type of ultrasound may also be used in late pregnancy. If you're nervous about a transvaginal ultrasound, here's what the equipment looks like and what happens during the procedure.

What happens during the procedure?

For Field’s second-trimester ultrasound, the technician first took a few minutes to explain the process and tell her how long it might take. “She put warm gel on my belly and then rubbed the transducer—it looks a bit like a microphone—over my belly. At the end, she called in my husband and showed us the baby’s head, heart and back, and told us that she couldn’t really see the baby’s sex because of the position,” explains Field.

How does it feel?

While an ultrasound is not painful, you may feel discomfort as the technician presses down against your belly or moves the transducer around in your vagina. Field says that she found it difficult to lie flat on her back for the full forty minutes it took to complete the scan. “I started to feel light-headed and had to ask to sit up and take a break at one point,” Field says.

What will the technician tell me?

“Not much,” says Field. “The results must be evaluated by your doctor or midwife and then presented to you. It can be hard to have the technician looking at the screen and not really say much, but that’s how it’s done.”

What if there are problems with the baby or placenta?

If there is a concern, you may learn about it in a few days or at your next appointment. Carolyn Van Delst of Chesterville, Ont., had an ultrasound with her second baby at 20 weeks. The technician didn’t say anything then, and she was surprised to get a phone call from her family doctor a few days later. “I was especially surprised because I’d been seeing a midwife during the pregnancy,” Van Delst says. “They told me I had a partial placenta previa (the placenta was partially covering the cervix).” By 36 weeks, however, the placenta had moved away from the cervix (as happens in the majority of cases).

In some cases, you’ll be told about concerns during the scan. During her third pregnancy, Van Delst underwent an ultrasound at 17 weeks. Partway through the exam, the technician said, “I’ll be right back,” and returned with a doctor. The doctor looked at the ultrasound images and pointed out some potential problems with the baby’s heart. “I have to admit I watch the technician’s face for any clues that she might be concerned because I know they wouldn’t tell me,” says Van Delst.

The possible heart problem also resolved itself, and Van Delst’s baby girl is now seven years old and perfectly healthy.

Sometimes, the ultrasound technician is not able to find a heartbeat and sees signs that your baby may have died in utero. Usually, in these situations, a doctor will be brought in immediately to confirm the diagnosis and discuss options with you. You may, in fact, have gone in for an ultrasound because you suspected this might be the case.

It’s also important to remember that an ultrasound has limitations. It can reveal some possible problems, but others may not be detected during the scan or may develop at a later date. Your ultrasound results may also give “false positives” that suggest problems that turn out not to be there.

Light’s recent ultrasound showed potential problems with her baby’s stomach, and she comments that this has added some stress to her pregnancy: “I had been looking forward to the ultrasound results in order to have some reassurance that the baby was fine, but instead I will be wondering and worrying about my baby’s health for the next few months.”

Do I have to have an ultrasound if I don’t want to?

The short answer is no. Many ultrasounds are done for fairly routine reasons—to confirm the baby’s due date or heartbeat, for example. Others are done to gain specific information. Discuss with your doctor or midwife the reasons he or she is recommending an ultrasound, and decide for yourself if you would like to have one done.

A previous version of this article was published on June 6, 2009.