Excerpt from the National Institutes of Health Consensus Development Conference Statement -- February 6-8, 1984
" Lack of risk has been assumed because no adverse effects have been
demonstrated clearly in humans. However, other evidence dictates that a
hypothetical risk must be presumed with ultrasound. Like-wise, the
efficacy of many uses of ultrasound in improving the management and outcome
of pregnancy also has been assumed rather than demonstrated, especially its
value as a routine screening procedure. "
WHAT TYPES OF ULTRASOUND SCANNING ARE CURRENTLY USED IN OBSTETRIC PRACTICE?
HOW EXTENSIVE IS THIS USE? WHAT IS KNOWN ABOUT THE DOSE/EXPOSURE TO THE
FETUS AND THE MOTHER FROM EACH TYPE?
On the basis of the collective experience of members of the panel, the
material presented, and the literature review that was conducted, we
conclude that in obstetric practice in the United States, use of diagnostic
ultrasound imaging has an expanding role, and its use is becoming
widespread. Information on the extent of use of diagnostic ultrasound in
pregnancy was available from single institutions and states, marketing
studies, the office survey conducted by the American College of
Obstetricians and Gynecologists, and the 1980 National Natality Survey.
These data lead to estimates of the percentage of pregnant women exposed to
at least one ultrasound examination ranging from a low of 15 percent to a
high of 40 percent. There is reason to believe that all of these data
sources seriously underestimate the true extent of exposure to ultrasound
since they do not necessarily include exposure via Doppler devices,
including those used to listen to fetal heart tones and in antepartum and
intrapartum fetal heart rate monitoring.
Exposure to imaging devices in the recent past has been to static scanners,
real-time equipment of the linear array type, and mechanical sector
scanners. The quantity used most often to report instrumentation output is
intensity. Typical time average value ranges of intensity are 0.1-60
mW/cm^2 (spatial average, temporal average intensity) and 1-200 mW/cm^2
(spatial peak, temporal average intensity). The spatial peak, pulse average
intensity typically ranges from 1-200 W/cm^2 for such pulsed ultrasound
The time average intensities of the typical obstetrical Doppler devices
used to listen to the fetal heart and for fetal heart rate monitoring in
the antepartum and intrapartum period are within the same range as for
pulsed equipment. These systems operate in the continuous wave mode, viz,
0.2-20 mW/cm^2 (spatial average, temporal average intensity) and 0.6-80
mW/cm^2 (spatial peak, temporal average intensity). As new technologies and
applications evolve, for example, measurement of blood flow using pulsed
Doppler, exposure levels may be substantially higher.
Manufacturers of ultrasound equipment introduced into U.S. commerce are
required to report outputs to the FDA. We recommend that these quantities
be measured and reported to the user in a form consistent with the
requirements of the AIUM/NEMA Safety Standard for Diagnostic Ultrasound
Dose is a quantitative measure of an agent that is given or imparted and
combines quantities such as intensity and exposure time. No dose quantity
has been identified for ultrasound. Variation in tissue properties between
individuals as well as scanning conditions influence dose in an
unpredictable way. For all practical purposes, fetal dose cannot be
quantitated precisely. For this reason, there are no data on the dose to
either the mother or the fetus in the clinical setting. Documentation of
dwell time and type of machine and transducer used would begin to address
this problem. It is recommended that at least this specific exposure
information be recorded for each examination. Thus, it is important that
each exposure to ultrasound by all Doppler and imaging devices be recorded.
WHAT ARE THE THEORETICAL RISKS OF ULTRASOUND TO THE FETUS AND THE MOTHER?
WHAT EVIDENCE EXISTS FROM ANIMAL, TISSUE CULTURE, AND HUMAN STUDIES ON THE
ACTUAL EXTENT OF THE RISK?
The panel conducted an extensive review of the primary literature on this
subject and of reports by the Bureau of Radiological Health (1976), Food
and Drug Administration (1982), World Health Organization (1982), and the
National Council on Radiation Protection and Measurements (1984).
A number of epidemiological studies tend to support the safety of
diagnostic ultrasound exposure in humans. In particular, in the three
randomized clinical trials in which half of the women were exposed
routinely to ultrasound, there was no association of routine ultrasound
exposure with birth weight. In the two studies that addressed the subject,
no association of ultrasound exposure with hearing loss was observed. On
the other hand, many of the studies reporting on the safety of diagnostic
ultrasound in humans were considered inadequate to address many other
important issues because of technical problems in conducting such research.
Some of the more than 35 published animal studies suggest that in utero
ultrasound exposure can affect prenatal growth. When teratological effects
have been found, energies capable of causing significant hyperthermia have
A number of biological effects have been observed following ultrasound
exposure in various experimental systems. These include reduction in immune
response, change in sister chromatid exchange frequencies, cell death,
change in cell membrane functions, degradation of macromolecules, free
radical formation, and reduced cell reproductive potential. It should be
noted that (a) some of the studies employed energy levels greater than
would be expected to exist in clinical use; (b) in vitro exposure
conditions to ultrasound used in many of the experiments are hard to place
in perspective for risk assessment; (c) some of the observations, for
example, sister chromatid exchange frequency changes and induction of
chromosomal abnormalities, have not been reproducible, tending to refute
the original findings. Nevertheless, some of the reported effects cannot be
ignored or overlooked and deserve further study as outlined in our answer
to Question 5. The existence of these studies is one of the factors that
contributed to our decision that routine ultrasound screening cannot be
recommended at this time.
BASED ON THE AVAILABLE EVIDENCE, WHAT ARE THE APPROPRIATE INDICATIONS FOR,
AND THE LIMITATIONS ON, USE OF ULTRASOUND IN OBSTETRICS TODAY?
From the body of information reviewed, taking into account the available
bioeffects literature, data on clinical efficacy, and with concern for
psychosocial, economic, and legal/ethical issues, it is the consensus of
the panel that ultrasound examination in pregnancy should be performed for
a specific medical indication. The data on clinical efficacy and safety do
not allow a recommendation for routine screening at this time.
Ultrasound examinations performed solely to satisfy the family's desire to
know the fetal sex, to view the fetus, or to obtain a picture of the fetus
should be discouraged. In addition, visualization of the fetus solely for
educational or commercial demonstrations without medical benefit to the
patient should not be performed.
Prior to an ultrasound examination, patients should be informed of the
clinical indication for ultrasound, specific benefit, potential risk, and
alternatives, if any. In addition, the patient should be supplied with
information about the exposure time and intensity, if requested. A written
form may expedite this process in some cases. Patient access to educational
materials regarding ultrasound is strongly encouraged. All settings in
which these examinations are conducted should assure patients' dignity and
Given that the full potential of diagnostic ultrasound imaging is
critically dependent on examiner training and experience, the panel
recommends minimum training requirements and uniform credentialing for all
physicians and sonographers performing ultrasound examinations. All health
care providers who use this modality should demonstrate adequate knowledge
of the basic physical principles of ultrasound, equipment, recordkeeping
requirements, indications and safety.
WHAT FURTHER STUDIES ARE NEEDED OF EFFICACY AND SAFETY OF USE OF ULTRASOUND
It is critical, in view of the existing data and the special considerations
affecting fetal and embryonic development, to encourage and support a
sustained research effort aimed specifically at test systems that can help
provide a better data base for developing reasonable estimates of
bioeffects and of risk. In particular, we recommend:
1. The study of fundamental mechanisms leading to bioeffects.
2. Laboratory experiments that focus especially on those cellular
processes that are most likely to be affected during embryonic and
3. Postnatal studies in animals after in utero exposure to ultrasound.
4. Exploration of interactions between administered ultrasound and such
developmentally significant agents as drugs, nutrition, ionizing
radiation, hyperthermia, and hypoxia.
5. Development of improved dosimetry.
A long-term followup of infants involved in a randomized clinical trial
would help clarify questions about the effect of ultrasound on development
in humans, and other epidemiologic studies using a wide variety of methods
should be considered. Studies of the psychosocial, ethical, and legal
aspects of ultrasound use are also needed.
Further nonexperimental studies that seek to establish the clinical
efficacy of ultrasound should address the question of its contribution to
reducing morbidity and mortality. Randomized, controlled clinical trials of
routine ultrasound screening in pregnancy should be conducted in the United