Conditions + Treatments

Prenatal Ultrasound - FAQ

LIke ThisLIke ThisLIke ThisLIke ThisLIke This

What is ultrasonography?

Ultrasound uses high frequency sound waves and their echoes to make cross-sectional images of the fetus. Ultrasound produces no radiation and is painless.

  • The sound waves go through your skin and reflect or "echo" in a different way off of each part of the body, such as bone, fluid, and soft tissue.
  • These echoes form a picture.
  • The technique is similar to the echolocation used by bats and whales and the SONAR that guides ships and submarines.

Your ultrasound exam may include fetal Doppler, which is a type of ultrasound that measures the flow of blood through blood vessels.

  • The movement of blood cells causes a change in pitch of the reflected sound waves (called the Doppler effect).
  • Waveforms of the blood flow are shown on the ultrasound screen.
  • A computer collects and processes the sounds and creates graphs or color pictures that represent the flow of blood through the blood vessels.

When are prenatal ultrasounds typically done?

We’ve put together a list of the times in your pregnancy that your doctor may prescribe an ultrasound as well as what kind of information those ultrasounds will provide.

In the first trimester:

  • To establish the dates of a pregnancy
  • To document heartbeat
  • To determine the number of fetuses and identify placental structures
  • To diagnose an ectopic pregnancy or miscarriage
  • To examine the uterus and other pelvic anatomy
  • To detect fetal abnormalities (in some cases)

Mid-trimester: (sometimes called the 18-to-20 week scan)

  • To confirm pregnancy dates
  • To determine the number of fetuses
  • To assist in prenatal procedures
  • To detect fetal abnormalities
  • To check the amount of amniotic fluid
  • To examine blood flow patterns
  • To assess fetal well-being
  • To examine the placenta
  • To measure the length of the cervix
  • To monitor fetal growth

Third trimester:

  • To monitor fetal growth
  • To check the amount of amniotic fluid
  • To assess the placenta
  • To assess fetal well-being
  • To determine fetal position

How is an ultrasound scan performed?

Although the specific details of each procedure vary slightly, ultrasounds generally follow this process.

Two types of ultrasounds can be performed during pregnancy:

  • Transabdominal ultrasound - Gel is applied to the abdomen and the ultrasound transducer glides over the gel on the abdomen and pelvis to create the image.
  • Transvaginal ultrasound - A transducer is inserted into the vagina and rests against the back of the vagina to create an image.
    • A transvaginal ultrasound produces a sharper image and is often used in early pregnancy.

What kind of equipment do we use to perform ultrasounds?

  • Ultrasound machines are about the size of a grocery cart.
  • A TV screen for viewing the images is attached to the machine.
  • The room used for scanning has dim lighting so that the pictures can be seen more clearly.
We are grateful to have been ranked #1 on U.S. News & World Report's list of the best children's hospitals in the nation for the third year in a row, an honor we could not have achieved without the patients and families who inspire us to do our very best for them. Thanks to you, Boston Children's is a place where we can write the greatest children's stories ever told.”
- Sandra L. Fenwick, President and CEO

Boston Children's Hospital 300 Longwood Avenue, Boston, MA 02115 617-355-6000 | 800-355-7944

Close