Pregnancy: Fetal ultrasounds

An ultrasound scan is a technique that uses sound waves to produce images of your baby in the womb. It’s a moment a parent never forgets!

First trimester ultrasound

The Society of Obstetricians and Gynaecologists of Canada recommends having an ultrasound 11 to 14 weeks into your pregnancy to determine how far along you are. The technician will measure the fetus’s crown-rump length (from the tip of the crown to the coccyx), which is directly related to the age of the fetus.

An ultrasound can be especially helpful if you can’t remember the exact date of your last period or if your menstrual cycle is irregular. Knowing the exact date of conception will give you a more accurate due date, which could help avoid unnecessary induction of labour. This ultrasound is more accurate if done between the 7th and 12th week of pregnancy.

Second trimester ultrasound

Your next fetal ultrasound will typically be performed between weeks 18 and 22 of pregnancy. Its main purpose is to identify any abnormalities in your baby’s limbs and organs (heart, liver, kidneys, stomach, bladder, brain, etc.) that might need to be addressed. It’s also used to check the following:

• Your baby’s age
• How many babies you’re carrying
• The position of the baby and placenta
• Your baby’s growth and development in comparison to age
• The length of your cervix

Are ultrasounds safe?

Fetal ultrasounds use low-energy sound waves. There is no evidence to suggest that ultrasounds have long-term negative effects (physical or cognitive) on humans. Ultrasounds have so many benefits that experts agree they are safe under the following conditions:

• The scans are done only when medically necessary, such as to establish the age of the fetus or to detect potential abnormalities.
• The machine is set at the lowest possible setting.
• The number and duration of ultrasound sessions are limited.

The Canadian Association of Radiologists and the Society of Obstetricians and Gynaecologists of Canada caution against using ultrasounds for the sole purpose of learning the sex of the baby or to produce keepsake pictures or videos. Doing so unnecessarily increases the baby’s exposure to ultrasound waves. Some studies, conducted in both humans and animals, have indicated that this form of energy may theoretically affect fetal development, according to the two groups’ joint policy statement.

Moreover, the Collège des médecins du Québec and the Ordre des technologues en imagerie médicale, en radio-oncologie et en électrophysiologie médicale du Québec published a joint policy statement in 2016. According to this statement, doctors should provide care or issue prescriptions only when medically necessary. “Entertainment” ultrasounds should therefore not be prescribed without a valid medical reason. Furthermore, medical imaging technicians will not be able to perform such ultrasounds without a medical prescription.

Things to keep in mind

• If performed between weeks 7 and 12 of your pregnancy, an ultrasound can help establish a more accurate due date.
• Ultrasounds are use to detect fetal abnormalities.
• Ultrasounds are safe if used only when medically necessary.

Scientific review: Dr. Bi Lan Wo, maternal-fetal medicine specialist, CHUM Research and copywriting: The Naître et grandir team Updated: September 2020