Prenatal ultrasound

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Your health care provider will offer you one or more ultrasound scans during your pregnancy. These prenatal ultrasound scans provide important medical information that will help you and your health care provider create the best care plan for you and your baby.

What is a prenatal ultrasound?

A prenatal ultrasound scan is a painless medical procedure. It is requested by your health care provider (such as your physician, midwife, or nurse practitioner). Most often, it will be recommended during your first and second trimesters.

It uses sound waves to produce a live image of your baby inside the uterus that can be seen on a monitor. These ultrasounds are done by certain health care providers who are trained and authorized to perform the care activity (such as physicians, and diagnostic medical sonographers). They use a hand-held instrument called a transducer that creates sound waves the human ear cannot hear. The ultrasound machine detects sound waves that bounce off the baby and turns them into an image.

Most often, medical ultrasounds are two-dimensional (2D). This creates a flat image rather than a three-dimensional (3D) image.

These images are used to assess the health and well-being of your baby. A radiologist reviews the images and sends a written report to your prenatal care provider. Your care provider will then review the findings with you and if it was done at a health authority facility then the ultrasound report will be available to you shortly after in Health Gateway.

How is the prenatal ultrasound done?

There are 2 ways to do a prenatal ultrasound:

  1. A transabdominal ultrasound is done by moving a transducer over your belly. This is the typical method used during routine prenatal ultrasounds
  2. An endovaginal (EV), or transvaginal, ultrasound is done by gently placing a narrow transducer in your vagina. This method provides a better view of the lower part of your uterus and is used to assess certain conditions that might arise in your pregnancy. If an endovaginal scan is needed, the person doing the ultrasound will discuss this with you. With this information, you can decide whether to consent.

Are prenatal ultrasounds safe?

Medical 2D ultrasounds have been used in pregnancy for over 50 years. Many studies have been done to determine if they are safe. Worldwide, prenatal ultrasounds have been performed with no confirmed health risks for the baby or the mother. 

What information will a prenatal ultrasound provide?

The scan will show different information depending on when the ultrasound is performed during your pregnancy and the reason your health care provider has requested it.

First trimester (up to 14 weeks)

An ultrasound done during the first trimester is often called a ‘dating ultrasound’ and is the best time to:

  • Determine how far along you are in your pregnancy
  • Estimate your due date
  • Help determine if there is more than one baby and if they share a placenta or the amniotic sac (the bag of amniotic fluid surrounding the baby inside the uterus)

Some people are eligible for a special type of 1st trimester ultrasound called a nuchal translucency (NT) ultrasound. This NT ultrasound is done along with blood tests to determine the chance of the baby having Down syndrome (DS) or Trisomy 18. Talk to your health care provider to learn more about your prenatal genetic screening options or visit Perinatal Services BC Prenatal Genetic Screening web page at www.perinatalservicesbc.ca/our-services/screening-programs/prenatal-genetic-screening.

Second trimester (14 to 27 weeks)

An ultrasound between 18 and 22 weeks into your pregnancy is often called the detailed, or anatomical ultrasound. It is the best time to:

  • Look at the anatomy of your baby and try to see if there are any growth concerns or birth defects. The shape and size of your baby’s physical structures are examined, but the ultrasound cannot always tell how well those structures are developing.
  • Look for ultrasound “markers”. Markers are not birth defects. They are most often normal variations in growth and development. When seen in combination, markers might suggest a congenital condition, such as Down syndrome or Trisomy 18. Your health care provider may offer further information and testing if markers are seen. Most babies that have an isolated ultrasound “marker” are completely healthy and well.

Ultrasound cannot detect all prenatal issues or conditions. Having a completely normal ultrasound does not guarantee a completely healthy pregnancy or baby. In addition to offering prenatal ultrasound, your health care provider will provide regular physical assessment and other screening tests to monitor you and your baby’s health throughout your pregnancy.

Third trimester (after 27 weeks)

Your health care provider may ask you to have additional ultrasounds in the 3rd trimester if they are concerned about:

  • Your baby’s size, growth, or position
  • The amount of amniotic fluid around your baby
  • The location of your placenta
  • The length of your cervix
  • When your pregnancy has gone beyond the due date, to check on the well-being of the fetus and to help with decisions about induction of labour

Can I find out the sex of my baby?

During the 2nd trimester detailed ultrasound, the genitals (sex organs) of your baby are usually viewed as part of the typical anatomy assessment.

The diagnostic medical sonographer performing the ultrasound is not able to give you detailed information about what they see on the ultrasound, including the sex of your baby. Only your health care provider who requested the ultrasound scan (such as your physician, midwife, or nurse practitioner) can provide the results of the scan. If you do not want to know the sex of your baby, let your health care provider and the diagnostic medical sonographer know.

Be aware that the ultrasound is not always correct in determining the sex of your baby. Also, you cannot book a medical ultrasound for the sole purpose of identifying the sex of the baby.

What factors might limit the information that I get from an ultrasound?

Some factors may limit what information can be reported from your ultrasound, such as:

  • Timing of the scan
  • The size or position of your baby
  • The location of your placenta
  • The volume of amniotic fluid around your baby
  • If you are pregnant with twins or multiples

If you have a high body mass index (BMI), can I get pictures from the 2nd trimester detailed ultrasound?

Some clinics that perform medically indicated 2D ultrasound scans do provide photos. When you check in for your ultrasound, ask about this possibility and the cost.

Some clinics offer optional, private-pay, 3D ultrasounds for keepsake or entertainment purposes.

Health Canada, the Society of Obstetricians and Gynaecologists of Canada, the Canadian Association of Radiologists, the College of Physicians and Surgeons of BC, and the International Society of Ultrasound and Gynecology recommend ultrasounds for medical purposes only. 

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