A balanced view of prenatal ultrasound would include information on the history, uses and potential effects of this technology. Parents should be as informed as possible in order to cooperate responsibly with their health care providers.
Safer Than X-rays?
As late as the 1970s, X-ray scans were used in prenatal obstetric practice for many of the same reasons that ultrasound scans are used today. A scan provides a relatively quick and reliable way to check the size and position of the fetus, to date the pregnancy, and to look for signs of abnormalities. While X-rays are best for obtaining static images of dense tissues, such as bone, ultrasound is much better for viewing details of soft, even moving, tissues.
When the widespread use of X-rays in prenatal care was correlated with a statistical rise in the rates of leukemia in young children, the routine use of X-rays in prenatal care dropped. Although low levels of X-ray radiation do not cause significant damage in most patients, rapidly developing fetal tissue is at greater risk. In the 1980s, ultrasound was hailed as a safe alternative to X-rays. To date, there are no reports of fetal deaths attributable to diagnostic ultrasound. However, this does not mean that ultrasound has no effects on living tissues.
How Ultrasound Works
Like sonar, ultrasonography uses high-frequency mechanical vibrations above the range that can be heard by the human ear to detect objects and form images. Although a fetus cannot hear ultrasound directly, secondary vibrations caused by ultrasound passing through bone may be audible, ranging from a sound akin to the highest note on a piano to a docking subway train, according to Eugenie Samuel in the December 4, 2001 New Scientist article "Fetuses can hear ultrasound examinations". This may explain empirical reports of fetuses becoming more active during ultrasound.
Diagnostic ultrasound scanners transmit vibrations best through fluid media, which is why gel is applied to the skin to improve conduction. Ultrasound waves can be partially blocked or reflected by dense tissue, which is why specially designed ultrasound wands are sometimes inserted into the vagina to obtain a clearer view inside the pelvic region. Changes in the waves as they "bounce" around tissues of varying densities are recorded by the scanner and translated into images.
As ultrasound waves move around, the vibrations may concentrate, causing increases in heat, pressure, or motion. Non-diagnostic ultrasound has been used to kill tumors through overheating and to shatter kidney stones. Therefore, the power level and duration of ultrasound is of significant importance.
Why Some Have Concerns
Currently, it is not possible to accurately measure all of ultrasound's effects on a fetus at the cellular level. Diagnostic ultrasound machines monitor the mechanical index, a measure of force, and the heat index, a measure of temperature, in order to minimize effects on fetal tissue. However, precise safety levels for prenatal ultrasound have not been scientifically determined and not all operators watch the indexes on the machine.
Some scientists have found that ultrasound vibrations can cause "cavitation" in living cells, in which tiny gas bubbles coalesce to form bigger bubbles. This can lead to the rupture of delicate cell structures and even cell death. Mice exposed to diagnostic levels of ultrasound have been found to have slower rates of learning and bleeding intestines. Comparative studies of children exposed to fewer or more prenatal scans have yielded mixed results, with some researchers finding evidence of brain damage and many doctors disputing such claims.
Further complicating the issue is that power levels for diagnostic ultrasound machines have increased eight-fold since the early 1990s. Studies of the effects of prenatal ultrasound prior to that date may therefore no longer be valid. Some scientists have speculated that the fetal brain may be especially vulnerable to damaging effects during periods of rapid growth and migration of neurons.
Does Ultrasound Save Lives?
Prenatal ultrasound increases the safety of procedures like amniocentesis and assists in the early detection of life-threatening complications during delivery. It is a powerful monitoring tool for obstetricians and other health professionals.
However, according to The RADIUS Study Group in the September 16, 1993 New England Journal of Medicine, there was no significant effect on outcome with the use of prenatal ultrasound. In other words, statistically speaking, ultrasound does not seem to make pregnancy and delivery any safer for baby, possibly because early detection of suspected fetal abnormalities increases the termination rate. As with any medical procedure, both risks and benefits should be considered on an individual basis.
Dana Caddis - I have always been interested in writing, and won awards for it in school. My second great interest is biology, which I studied in ...
