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Diagnostic Ultrasound Imaging in Pregnancy

National Institutes of Health
Consensus Development Conference Statement
February 6-8, 1984

Conference artwork depicting a stylized ultrasound of two fetuses within the abdomen .

This statement is more than five years old and is provided solely for historical purposes. Due to the cumulative nature of medical research, new knowledge has inevitably accumulated in this subject area in the time since the statement was initially prepared. Thus some of the material is likely to be out of date, and at worst simply wrong. For reliable, current information on this and other health topics, we recommend consulting the National Institutes of Health's MedlinePlus http://www.nlm.nih.gov/medlineplus/.

This statement was originally published as: Diagnostic Ultrasound Imaging in Pregnancy. NIH Consens Statement 1984 Feb 6-8;5(1):1-16.

For making bibliographic reference to the statement in the electronic form displayed here, it is recommended that the following format be used: Diagnostic Ultrasound Imaging in Pregnancy. NIH Consens Statement Online 1984 Feb 6-8 [cited year month day];5(1):1-16.

Introduction

From crude initial studies in the 1950s, ultrasonography in pregnancy has become a highly developed technology capable of detecting many fetal structural and functional abnormalities. It has found application in detecting ectopic pregnancy and multiple pregnancy, assessing fetal life and function, diagnosing physical anomalies, and guiding physicians as they make efforts to treat the fetal patient. The advent of ultrasound has overcome many of the diagnostic limitations of X-ray and has virtually eliminated the need for fetal exposure to ionizing radiation.

With these advantages and marked improvements in the technology and equipment, the use of ultrasound in obstetric practice has grown rapidly. The procedure is available in nearly all hospitals, and many physicians have acquired equipment for use in their offices. Further, because of the absence of clinically perceived risk of ultrasound and its usefulness in assessing structural anomalies, multiple pregnancy, and fetal size and gestational age, many practitioners have begun to advocate its routine use as a screening device in all pregnancies.

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.

The marked increase in the use of ultrasound, coupled with concerns regarding its safety and efficacy, prompted three NIH components--the National Institute of Child Health and Human Development (NICHD), the Office of Medical Applications of Research (OMAR), and the Division of Research Resources (DRR)--and the FDA National Center for Devices and Radiological Health to join in sponsoring a
Consensus Development Conference to assess the use of diagnostic ultrasound imaging in pregnancy. The conference was held on February 6-8, 1984, after a year of preparation by the panel. After presenting a preliminary report at the conference, hearing the testimony of experts, and receiving comments and criticisms from the medical/scientific community, as well as from the public at large, the panel, consisting of physicians, basic scientists, epidemiologists, nurses, educators, sonographers, and public representatives, considered all of the information received and provided answers to the following questions that were posed to the panel:

  1. 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?
  2. For what purposes is ultrasound now used in pregnancy? For each use, what is the evidence that ultrasound improves patient management and/or outcome of pregnancy?
  3. 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?
  4. Based on the available evidence, what are the appropriate indications for, and limitations on, the use of ultrasound in obstetrics today?
  5. What further studies are needed of efficacy and safety of use of ultrasound in pregnancy?
 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/cm2 (spatial average, temporal average intensity) and 1-200 mW/cm2 (spatial peak, temporal average intensity). The spatial peak, pulse average intensity typically ranges from 1-200 W/cm2 for such pulsed ultrasound equipment.

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/cm2 (spatial average, temporal average intensity) and 0.6-80 mW/cm2 (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 Equipment.

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.  

For What Purposes Is Ultrasound Now Used in Pregnancy? For Each Use, What Is the Evidence That Ultrasound Improves Patient Management and/or Outcome of Pregnancy?

Ultrasound has been used in a wide variety of clinical situations to aid in managing pregnancy. For each of these applications, there is literature recording the clinical experience from various centers, with evidence of benefits ultrasound has had in each respective application, although these applications have not been subjected to the rigorous evaluation provided by a randomized, controlled clinical trial. The following should not be considered circumstances in which use of diagnostic ultrasound imaging is mandatory. Rather, where significant clinical questions exist, the resolution of which would alter the remainder of prenatal care, ultrasound can be of benefit for:

  • Estimation of gestational age for patients with uncertain clinical dates, or verification of dates for patients who are to undergo scheduled elective repeat cesarean delivery, indicated induction of labor, or other elective termination of pregnancy. Ultrasonographic confirmation of dating permits proper timing of cesarean delivery or labor induction to avoid premature delivery.
  • Evaluation of fetal growth (e.g., when the patient has an identified etiology for uteroplacental insufficiency, such as severe pre-eclampsia, chronic hypertension, chronic renal disease, severe diabetes mellitus, or for other medical complications of pregnancy where fetal malnutrition, i.e., IUGR or macrosomia, is suspected). Following fetal growth permits assessment of the impact of a complicating condition on the fetus and guides pregnancy management.
  • Vaginal bleeding of undetermined etiology in pregnancy. Ultrasound often allows determination of the source of bleeding and status of the fetus.
  • Determination of fetal presentation when the presenting part cannot be adequately determined in labor or the fetal presentation is variable in late pregnancy. Accurate knowledge of presentation guides management of delivery.
  • Suspected multiple gestation based upon detection of more than one fetal heartbeat pattern, or fundal height larger than expected for dates, and/or prior use of fertility drugs. Pregnancy management may be altered in multiple gestation.
  • Adjunct to amniocentesis. Ultrasound permits guidance of the needle to avoid the placenta and fetus, to increase the chance of obtaining amniotic fluid, and to decrease the chance of fetal loss.
  • Significant uterine size/clinical dates discrepancy. Ultrasound permits accurate dating and detection of such conditions as oligohydramnios and polyhydramnios, as well as multiple gestation, IUGR, and anomalies.
  • Pelvic mass detected clinically. Ultrasound can detect the location and nature of the mass and aid in diagnosis.
  • Suspected hydatidiform mole on the basis of clinical signs of hypertension, proteinuria, and/or the presence of ovarian cysts felt on pelvic examination or failure to detect fetal heart tones with a Doppler ultrasound device after 12 weeks. Ultrasound permits accurate diagnosis and differentiation of this neoplasm from fetal death.
  • Adjunct to cervical cerclage placement. Ultrasound aids in timing and proper placement of the cerclage for patients with incompetent cervix.
  • Suspected ectopic pregnancy or when pregnancy occurs after tuboplasty or prior ectopic gestation. Ultrasound is a valuable diagnostic aid for this complication.
  • Adjunct to special procedures, such as fetoscopy, intrauterine transfusion, shunt placement, in vitro fertilization, embryo transfer, or chorionic villi sampling. Ultrasound aids instrument guidance, which increases safety of these procedures.
  • Suspected fetal death. Rapid diagnosis enhances optimal management.
  • Suspected uterine abnormality (e.g., clinically significant leiomyomata, or congenital structural abnormalities, such as bicornuate uterus or uterus didelphys, etc.). Serial surveillance of fetal growth and state enhances fetal outcome.
  • Intrauterine contraceptive device localization. Ultrasound guidance facilitates removal, reducing chances of IUD-related complications.
  • Ovarian follicle development surveillance. This facilitates treatment of infertility.
  • Biophysical evaluation for fetal well-being after 28 weeks of gestation. Assessment of amniotic fluid, fetal tone, body movements, breathing movements, and heart rate patterns assists in the management of high-risk pregnancies.
  • Observation of intrapartum events (e.g., version/extraction of second twin, manual removal of placenta, etc.). These procedures may be done more safely with the visualization provided by ultrasound.
  • Suspected polyhydramnios or oligohydramnios. Confirmation of the diagnosis is permitted, as well as identification of the cause of the condition in certain pregnancies.
  • Suspected abruptio placentae. Confirmation of diagnosis and extent assists in clinical management.
  • Adjunct to external version from breech to vertex presentation. The visualization provided by ultrasound facilitates performance of this procedure.
  • Estimation of fetal weight and/or presentation in premature rupture of membranes and/or premature labor. Information provided by ultrasound guides management decisions on timing and method of delivery.
  • Abnormal serum alpha-fetoprotein value for clinical gestational age when drawn. Ultrasound provides an accurate assessment of gestational age for the AFP comparison standard and indicates several conditions (e.g., twins, anencephaly) that may cause elevated AFP values.
  • Followup observation of identified fetal anomaly. Ultrasound assessment of progression or lack of change assists in clinical decisionmaking.
  • Followup evaluation of placenta location for identified placenta previa.
  • History of previous congenital anomaly. Detection of recurrence may be permitted, or psychologic benefit to patients may result from reassurance of no recurrence.
  • Serial evaluation of fetal growth in multiple gestation. Ultrasound permits recognition of discordant growth, guiding patient management and timing of delivery.
  • Evaluation of fetal condition in late registrants for prenatal care. Accurate knowledge of gestational age assists in pregnancy management decisions for this group.

 

The information presented in the material reviewed by the panel, including the studies of Bennett, Eik-Nes, Bakketeig, Grennert, and others, allowed no consensus that routine ultrasound examinations for all pregnancies improved perinatal outcome or decreased morbidity or mortality. There was, however, evidence that there was a higher rate of detection of twins and congenital malformations, as well as more accurate dating of pregnancy, but without significant evidence of improved outcome. The evidence with respect to the number of antepartum days of hospitalization and induction rates was contradictory among trials. The data on perinatal outcome were inconclusive. The panel recognized the inadequacy of the clinical trials on which these conclusions are drawn. Furthermore, it is acutely aware of the difficulty associated with conducting ideally controlled clinical trials and the large numbers of patients that must be included to uncover differences between control and experimental groups, where a morbid event occurs infrequently and spontaneously in the control population.

The panel concludes that diagnostic ultrasound for pregnant women improves patient management and pregnancy outcome when there is an accepted medical indication. Randomized, controlled clinical trials would be the best way in the United States to determine the efficacy of routine screening of all pregnancies.  

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 usually existed.

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 privacy.

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 in Pregnancy?

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 fetal development.
  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 States.  

Consensus Development Panel

Fredric Frigoletto, M.D. (Chairman)
Chief, Maternal Fetal Medicine
Department of Obstetrics and Gynecology
Brigham and Women's Hospital
Professor of Obstetrics and Gynecology
Harvard Medical School
Boston, Massachusetts
Robert Auerbach, Ph.D.
Professor of Zoology
University of Wisconsin
Madison, Wisconsin
Alexander Brickler, M.D.
Family Practitioner, Obstetrics and Gynecology
Tallahassee, Florida
Kenneth R. Gottesfeld, M.D.
Clinical Professor of Obstetrics, Gynecology, and Radiology
University of Colorado Health Sciences Center
Rose Medical Center
Denver, Colorado
Charles Hohler, M.D.
Director of Perinatology and Perinatal Ultrasound
Division of Reproductive Medicine
St. Joseph's Hospital and Medical Center
Phoenix, Arizona
Michael L. Johnson, M.D.
Director, Division of Diagnostic Ultrasound
Associate Professor of Medicine and Radiology
University of Colorado Health Sciences Center
Denver, Colorado
Jean H. Lea, M.P.H., R.D.M.S.
Associate Professor
College of Allied Health
University of Oklahoma
Oklahoma City, Oklahoma
Rosanna L. Lenker, B.S.N., C.N.M., C.F.N.P.
Instructor
Department of Obstetrics and Gynecology
Pennsylvania State University
School of Medicine
Milton S. Hershey Medical Center
Hershey, Pennsylvania
George A. Little, M.D.
Professor and Chairman
Department of Maternal and Child Health
Dartmouth Medical School
Hanover, New Hampshire
William D. O'Brien, Jr., Ph.D.
Professor of Electrical Engineering and of Bioengineering
Bioacoustics Research Laboratory
University of Illinois
Urbana, Illinois
Diana Petitti, M.D.
Assistant Professor
Division of Family and Community Medicine
University of California at San Francisco
School of Medicine
San Francisco, California
John T. Queenan, M.D.
Professor and Chairman
Department of Obstetrics and Gynecology
Georgetown University School of Medicine
Washington, D.C.
Karen Rothenberg, M.P.A., J.D.
Assistant Professor
University of Maryland School of Law
Baltimore, Maryland
Thomas H. Shepard, M.D.
Professor of Pediatrics
Head, Central Laboratory for Human Embryology
Department of Pediatrics
University of Washington
Seattle, Washington

Speakers

Robert Auerbach, Ph.D.
"The Risks of Ultrasound Use in Pregnancy: Data from In Vitro Studies"
Fredric Frigoletto, M.D.
"Preliminary Conclusions and Recommendations on the Appropriate Indications for and Limitations on Use of Diagnostic Ultrasound Imaging in Pregnancy Today"
Howard Hoffman, M.A.
"NCHS Data on Ultrasound Use"
Charles Hohler, M.D.
"The Uses and Benefits of Ultrasound in Pregnancy for Medical Indications"
William D. O'Brien, Jr., Ph.D.
"The Biophysics of Ultrasound: Types of Scanning, Dose/Exposure, and Extent of Use"
Diana Petitti, M.D.
"The Uses and Benefits of Ultrasound in Pregnancy for Routine Screening"
John T. Queenan, M.D.
"Research Needs"
Karen Rothenberg, M.P.A., J.D.
"Other Considerations: Relations with Patients, Training, and Ethical-Legal Issues"
Thomas H. Shepard, M.D.
"The Risks of Ultrasound Use in Pregnancy: Data from Animal and Human Studies"

Planning Committee

Mortimer B. Lipsett, M.D. (Chairman)
Director
National Institute of Child Health and Human Development
National Institutes of Health
Bethesda, Maryland
Duane Alexander, M.D.
Deputy Director
National Institute of Child Health and Human Development
National Institutes of Health
Bethesda, Maryland
James Benson
Deputy Director
National Center for Devices and Radiological Health
Food and Drug Administration
Rockville, Maryland
Heinz Berendes, M.D.
Director
Epidemiology and Biometry Research Program
National Institute of Child Health and Human Development
National Institutes of Health
Bethesda, Maryland
Michael J. Bernstein
Director of Communications
Office of Medical Applications of Research
National Institutes of Health
Bethesda, Maryland
Fredric Frigoletto, M.D.
Chief, Maternal Fetal Medicine
Department of Obstetrics and Gynecology
Brigham and Women's Hospital
Professor of Obstetrics and Gynecology
Harvard Medical School
Boston, Massachusetts
James G. Hill
Chief, Office of Planning and Evaluation
National Institute of Child Health and Human Development
National Institutes of Health
Bethesda, Maryland
Itzhak Jacoby, Ph.D.
Deputy Director
Office of Medical Applications of Research
National Institutes of Health
Bethesda, Maryland
Anne Krey
Genetics and Teratology Section
Center for Research for Mothers and Children
National Institute of Child Health and Human Development
National Institutes of Health
Bethesda, Maryland
Michaela P. Richardson
Office of Research Reporting
National Institute of Child Health and Human Development
National Institutes of Health
Bethesda, Maryland
Stephen W. Smith, Ph.D.
Deputy Chief, Acoustics Branch
National Center for Devices and Radiological Health
Food and Drug Administration
Rockville, Maryland
Melvin E. Stratmeyer, Ph.D.
Chief, Acoustic Radiation Branch
Division of Risk Assessment
Office of Radiological Health
National Center for Devices and Radiological Health
Food and Drug Administration
Rockville, Maryland

Conference Sponsors

National Institute of Child Health and Human Development
Mortimer B. Lipsett, M.D.
Director
Office of Medical Applications of Research, NIH
J. Richard Crout, M.D.
Director
Division of Research Resources, NIH
Betty H. Pickett, Ph.D.
Director
National Center for Devices and Radiological Health, FDA
John Villforth
Director

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