Skip to contentSkip to navigation

Third-trimester obstetrical ultrasound

A third trimester ultrasound scan generally provides information on fetal growth and well-being as well as placental function. In some circumstances, it can also diagnose malformations that were not visible on a second trimester ultrasound. It can be performed between the 26th and 40th weeks of pregnancy, but is more frequently done between the 28th and 34th weeks. This ultrasound is non-invasive since the technologist scans the surface of the mother’s belly with a probe.

The examination usually measures the following:

  • Biparietal diameter (one side of the head to the other)
  • Head circumference
  • Abdominal circumference
  • Femur length
  • Estimated fetal weight
  • Cardiac activity
  • Umbilical cord Doppler (in some cases)
  • Cerebral artery Doppler (in some cases)
  • Amniotic fluid quantity
  • Fetal well-being (biophysical profile)

Unlike first and second trimester ultrasounds, for which the patient’s bladder must be full, no preparation is required to perform a third trimester ultrasound. However, it is not advisable to apply cream to the abdomen during the 48 hours prior to the ultrasound, as this may negatively affect ultrasound transmission and image quality.

Preparing for the examination

No preparation is required before a third-trimester ultrasound.

May I be accompanied by someone for my appointment?

Even during the pandemic, a person of your choice can accompany you during this exam. If you come accompanied, be informed that this person will have to answer the same screening questionnaire as you will have to, and access could be refused. 

A medical prescription is mandatory to undergo this test. At Imagix™, third-trimester ultrasounds are interpreted by a gynecologist. Therefore, they are not covered by the Régie de l’assurance maladie du Québec (RAMQ), which only covers the cost of ultrasounds interpreted by a radiologist.

If your health care professional has prescribed this test for you or you have any questions, feel free to contact us at 1 833 590-2715.