Fetal ultrasound is a safe, painless and non-invasive test which provides you and your doctor with a first glimpse of your developing baby. It has many other uses during pregnancy.
Ultrasound was developed from radar technology more than 30 years ago and is safe. High-frequency sound waves are generated from a hand-held device called a transducer, which is held against the skin over the uterus. The sound waves pass through muscle, bone and the internal organs of the mother and her baby, and are reflected off these tissues in different ways. The returning waves are transformed into a visual image, in which fluid, fat, muscle or bone all have a different appearance. The images can be captured in “real-time” to show movement, for example the baby’s hand opening and closing, or the heart beating. Or the images can be presented as still pictures.
Ultrasound does NOT use ionizing radiation, unlike X-rays, and the energy levels of the sound waves as used for scanning of a baby are not great enough to alter atoms and molecules or damage biological tissues. However, at somewhat higher exposure levels, given daily for weeks at a time, ultrasound is used to speed the healing of bone fractures. And at much higher exposure levels, ultrasound produces a heating effect in tissue which is useful in treating sprains and pulled muscles.
Laboratory studies have shown that even diagnostic levels of ultrasound can produce physical effects in tissue, such as mechanical vibrations and a slight rise in temperature. In experiments with high-intensity waves it has also been shown to produce small pockets of gas in body fluids or tissues. There is no evidence that these physical effects can harm the fetus, but public health experts agree that there should always be a medical indication for performing an ultrasound, and that it should not be used casually or for prolonged periods of time during pregnancy unless absolutely essential.
- Used early in pregnancy to confirm that it is in the uterus, rather than in the tube.
- Used to date a pregnancy when the last period is unknown, or to confirm twins.
- Used to identify or follow the growth of ovarian cysts or fibroids in the uterus.
- Sometimes used to measure the length of the cervix, especially in women who have a history of pre-term labor in a prior pregnancy, or have had surgery to the cervix such as a cone biopsy or LEEP.
- Performed between 11-14 weeks to screen for chromosomal abnormalities such as Down Syndrome (called the Nuchal Translucency test).
- Typically performed on all women between 18 – 20 weeks of pregnancy when it may identify fetal abnormalities, especially of the brain, heart and bowel.
- Often used to identify the sex of the baby, but false diagnoses can be made.
- Used to determine location and development of the placenta, especially after bleeding.
- Used to determine a baby’s size and growth rate, and the position of the baby (breech or head down).
- Used to check for fetal health and well-being by assessing breathing and how the baby moves, as well as the amount of amniotic fluid around the baby.
- In special circumstances used to determine the amount of blood flow going to the baby.
Types of Ultrasounds
- Standard – these are the black and white images of the baby which most women are familiar with in pregnancy. In early pregnancy the whole of the uterus can be seen with the small fetus inside, and the length from the top of the head to the rump is used to date the pregnancy. Later on special measurements are taken of the length of the thigh bone, the circumference of the abdomen over the liver and stomach, and the circumference of the head, to calculate an estimated weight of the baby.
- Advanced or Targeted – these are used to further investigate a suspected abnormality identified by a standard ultrasound. For example, an echocardiogram may be done to look at the four chambers of the baby’s heart, and the way in which the great vessels emerge from the heart to go to the lungs and body.
- Doppler – measures slight changes in the frequency of the ultrasound waves as they bounce off moving objects, such as blood cells flowing in the blood. This can be used to see if a baby is anemic because the flow of blood in an anemic baby is faster than normal flow. It can also be used to see if placental blood vessels are sitting in front of the baby’s head. It can also be used to look at whether the arteries to the mother’s uterus are constricted, as sometimes occurs in pre-eclampsia (elevated blood pressure, specific to pregnancy). And it can be used to check the amount of blood flow in the umbilical cord of babies who are not growing as well as they should be (intra-uterine growth restriction).
- Three-dimensional (3D) – offers 3-D images with a high level of detail. This may be important for certain abnormalities to detect the full extent of the problem, for example brain abnormalities or facial deformities such as a cleft lip. However, just as with normal 2-dimensional ultrasound, it has the potential for misdiagnosis and errors in assessment.
This office does not perform 3-D sonography for “Keepsake” Images. In fact, a large body of medical opinion, including the FDA, considers this an unapproved use of a medical device.
How To Prepare
- Wear comfortable, loose-fitting clothing.
- Most likely the test needs to be done with a full bladder. For transvaginal ultrasound or those in late pregnancy, a full bladder usually isn’t necessary.
What To Expect
- The examination usually takes less than 30 minutes, and is done by a professional sonographer.
- It is painless, although transvaginal sonography requires a thin, covered probe to be inserted into the vagina against the cervix.
- You are usually positioned on an examination table and warm clear gel is applied to the abdomen. This improves conduction of sound waves and eliminates air pockets between your skin and the transducer, the small device that sends out the sound waves.
- The sonographer moves the transducer back and forth over the abdomen, directing the sound waves into the uterus and capturing them as they are reflected back.
- You look at the visual image on a screen.
What Affects The Test
Fetal ultrasound results may be affected by:
- Being very overweight or not being able to lie still during the procedure.
- An abnormally low amount of amniotic fluid preventing fetal movement and adequate visualization of the baby.
- Some fetal positions, for example if the head is very low in the pelvis.
- A very active fetus.
What You Should Know
A normal ultrasound result does not guarantee a normal, healthy baby. Not all birth defects are visible.Additional tests or procedures may be necessary if the results of your fetal ultrasound are not normal, for example amniocentesis.Your husband or partner is encouraged to be present during the test, and a photograph will generally be given to you.Your expected date of delivery (“due date”) may be changed based on an ultrasound done in early pregnancy. In fact, when done in the first 12 weeks, it is more accurate than using the last menstrual period.In the third trimester, fetal ultrasound does not accurately determine fetal age or weight.The effects of prolonged fetal ultrasound exposure have not been determined, and the U.S. Food and Drug Administration (FDA) does not recommend fetal ultrasound for nonmedical reasons, such as for only identifying the sex of the baby or for a personal keepsake.Three-dimensional (3-D) fetal ultrasound is being tested for use in evaluating certain fetal abnormalities, but is not yet widely available.
Our practice is accredited by the American Institute of Ultrasound in Medicine.