Is Pregnancy Ultrasound Safe for Your Baby? A Complete Guide to the Research

Table of Contents

• Key Points
• Background: Why This Question Matters
• How Ultrasound Works
• Recommendations from Regulatory Bodies
• Proposed Biological Effects of Ultrasound
• Thermal Effects: Heating of Tissues
• Mechanical Effects: Cavitation and Other Forces
• Evidence for Thermal Effects
• Evidence for Mechanical Effects
• Clinical Studies in Humans
• Nonrandomized Clinical Studies
• Randomized Controlled Trials
• Long-Term Outcomes
• Conclusions
• What This Means for Patients
• Limitations of the Evidence
• Recommendations
• Frequently Asked Questions
• Source Information

Key Points

• Medically indicated ultrasound performed by trained professionals appears safe; most studies found no link to childhood cancer, birth defects, or impaired development.
• Modern machines can produce much higher energy than older machines used in most safety studies, so the safety of current equipment is not fully established.
• The safety of Doppler, 3-D, and 4-D ultrasound, including keepsake imaging, has not been confirmed by long-term clinical studies.
• The person performing the ultrasound is responsible for safety; surveys found many practitioners have poor knowledge about ultrasound safety in pregnancy.
• The thermal index (TI) and mechanical index (MI) are displayed on modern machines; a value less than one is generally accepted as safe.

Background: Why This Question Matters

Ultrasound is one of the most widely used imaging tools in medicine, and it is especially important in pregnancy care. Most patients and doctors regard ultrasound as safe for the unborn baby. However, scientists have proposed two ways that ultrasound energy might harm tissue: by heating it (thermal effects) and by mechanical forces (nonthermal effects).

Most of the studies that support the safety of ultrasound were performed with machines that produced only a fraction of the energy that modern machines can produce. The safety of today's more powerful machines has not been confirmed by clinical studies. This gap in knowledge is the central concern of this review.

Doppler ultrasound is also being used more and more often in pregnancy. Doppler ultrasound can reach significantly higher output levels than standard B-mode imaging (the standard two-dimensional black-and-white picture). The long-term safety profile of Doppler ultrasound is unknown.

Another trend raises concern: the growing use of 3-dimensional (3-D) and 4-D ultrasound, including "keepsake" imaging requested by the public for non-medical reasons. These practices have not been proven safe.

Currently, the person performing the ultrasound is responsible for monitoring the safety of each exam. This is a problem, because recent surveys of ultrasound practitioners found that most end-users have poor knowledge about ultrasound safety in pregnancy (Sheiner et al., 2007; Marsal, 2005).

How Ultrasound Works

Ultrasound uses the properties of sound waves and body tissues to produce a two-dimensional image for clinical evaluation. The transducer (the handheld probe) contains piezoelectric crystals. These crystals emit and receive ultrasound waves after they are stimulated by an electrical current.

Different tissues reflect echoes differently. These differences in reflection determine the image that the machine produces. The operator then selects different modes depending on the clinical information needed. These modes include B-mode (brightness), M-mode (motion), and Doppler modes.

Recommendations from Regulatory Bodies

Several organizations and committees regulate ultrasound equipment and issue recommendations about its safety. Since 1976, the Food and Drug Administration (FDA) has set an upper limit for the output capacity of ultrasound equipment. The original limit was 94 mW/cm² (milliwatts per square centimeter).

In 1993, the maximum permissible output was increased almost eightfold, to 720 mW/cm². This change allowed significantly higher output potential. It also shifted responsibility away from strict regulation and toward the user's clinical decision-making at the time of the exam.

In addition to the FDA, several other organizations review current evidence and recommend clinical applications. These organizations include:

• The American Institute of Ultrasound in Medicine (AIUM)
• The World Federation for Ultrasound in Medicine and Biology (WFUMB)
• The European Federation of Societies for Ultrasound in Medicine and Biology (EFSUMB)
• The Australian Society for Ultrasound in Medicine (ASUM)

Proposed Biological Effects of Ultrasound

Scientists have proposed two biological mechanisms that could result from ultrasound exposure: thermal effects and nonthermal (mechanical) effects (Abramowicz et al., 2008; Hershkovitz et al., 2002).

In 1992, the AIUM, together with the American Institute of Ultrasound in Medicine and the National Electrical Manufacturer's Association (1998), developed the output display standard (ODS). The ODS gives the ultrasound user an estimate of the safety of the exam. The FDA supports the ODS. The ODS includes two parameters: the thermal index (TI) and the mechanical index (MI).

Thermal Effects: Heating of Tissues

Ultrasound energy is converted into heat at the tissue level. This heat can raise the temperature inside cells above normal body levels. The amount of energy absorbed depends on three things: the type of tissue, the duration of exposure, and the ultrasound mode or route used.

Hyperthermia (elevated body temperature) can cause birth defects, and doctors cannot directly measure temperature increases in a fetus. Because of these two facts, engineers developed a measure of thermal output for display on ultrasound equipment. This measure is the thermal index (TI).

The TI is a unitless approximation of the thermal risk being produced during an ultrasound exam. The machine calculates the TI in real time. A TI less than one is generally accepted as safe. The machine will display the TI if the capability of the machine exceeds one.

The TI is further divided into indices based on which tissue is exposed:

• TIS — soft tissue
• TIC — cranial bone (skull)
• TIB — bone

The user can view each of these indices on the machine, although only a single index may be displayed on the screen at one time. The WFUMB has recommended that a temperature elevation up to 1.5°C is the upper threshold for clinical use (Barnett et al., 2000).

Mechanical Effects: Cavitation and Other Forces

Nonthermal effects, also called mechanical effects, are another potential mechanism of harm during ultrasound exams. These effects refer to the relationship between interfaces, such as ultrasound waves and gas bubbles, within tissue (Stratmeyer et al., 2008).

Cavitation is the term for a biological effect that can result from that interaction (Lee and Frizzell, 1988). Ultrasound waves can start movement in and around gas bubbles. This movement may mechanically disturb nearby cells, or it may cause the bubbles to expand and collapse.

The ODS gives ultrasound users an approximation of this mechanical strain: the mechanical index (MI). Like the TI, a value less than one suggests a relative measure of safety.

Evidence for Thermal Effects

Much of the available evidence about thermal effects comes from laboratory and animal studies (Miller and Ziskin, 1989). Edwards has shown multiple teratogenic effects (birth defects) in the offspring of mammals exposed to high temperatures in the womb. These effects include problems of the musculoskeletal system, heart, kidneys, and central nervous system (Edwards, 1986; Graham, 2005).

These findings are less established in humans, but some human studies have proposed them (Chambers et al., 1998; Church and Miller, 2007; Layde et al., 1980; Medveczky et al., 2004). Edwards has suggested that the developing central nervous system is especially susceptible to injury from high temperature (Edwards, 2006).

A prospective cohort study in 1992 suggested an increased risk of neural tube defects among the offspring of women exposed to elevated temperatures. Neural tube defects are serious birth defects of the brain and spine. The elevated temperatures came from a variety of causes. Other studies have reflected this association (Milunsky et al., 1992). A recent meta-analysis reported a relative risk of 1.95 for neural tube defects in pregnancies complicated by maternal hyperthermia. A meta-analysis is a study that combines results from multiple studies. The 95% confidence interval [CI] was 1.30–2.92 (Moretti et al., 2005).

These studies represent the potential for thermal effects in the first trimester. Researchers have also considered effects at later stages of pregnancy. One area of particular interest is the potential for ultrasound energy to raise the temperature of bone, especially cranial bone (skull). This interest comes from the increasing use of middle cerebral artery Doppler in clinical practice and from the higher absorption potential of bone compared to other tissues (Barnett, 2001).

In 1998, one study measured temperature increases inside the brains of fetal guinea pigs exposed to ultrasound levels similar to clinical Doppler exposure. After 2 minutes of continuous exposure, temperatures increased by up to 4.9°C. Further work within that same dataset proposed that the TIB underestimated the actual measured temperatures (Horder et al., 1998a).

The WFUMB international recommendation states that fetal temperature elevations of 4°C above baseline after 5 minutes may be harmful (Barnett et al., 2000). This study provides an example of that potential.

Evidence for Mechanical Effects

Concerns about nonthermal bioeffects from ultrasound arose from animal research that documented lung hemorrhage (bleeding in the lungs) in animals after birth. In 1990, Child and colleagues reported hemorrhage in the lungs of mice exposed to ultrasound levels that are conceivable for diagnostic ultrasound equipment (Child et al., 1990).

Other animal studies have repeated these findings. These studies included monkeys, pigs, and rabbits (Baggs et al., 1996; Frizzell et al., 1994; O'Brien and Zachary, 1994; Tarantal and Canfield, 1994; Zachary and O'Brien, 1995).

The application of these findings to fetuses is unclear, because fetal lungs contain amniotic fluid instead of air. A mechanical effect of ultrasound was suggested in 1999, when hemorrhage was documented in the skulls of fetal mice exposed to 1.2 MHz of pulsed ultrasound (Dalecki et al., 1999). In 2007, researchers acknowledged the possibility of heating or cavitary mechanisms in the fetal hemorrhages noted in their study (Bigelow et al., 2007).

In 2006, researchers documented alterations in the migration of cerebral neurons (brain cells) within the brains of mice exposed to ultrasound. Neurons were labeled before ultrasound exposure ranging from 5 to 420 minutes. Delays in their migration were significant in animals exposed for 30 minutes or longer. Researchers hypothesized mechanical effects, although the mechanism is poorly understood and the overall clinical impact is unknown (Ang et al., 2006).

Clinical Studies in Humans

Researchers have evaluated a variety of clinical parameters in infants exposed to ultrasound. They identified these studies by searching PubMed and Medline using key search words. These words included obstetrical ultrasound safety, TI, MI, ODS, and diagnostic ultrasound exposure. The researchers also reviewed the bibliographies of identified studies to supplement their findings.

Table 1 of the original article shows the major nonrandomized clinical studies. Table 2 shows the randomized controlled trials.

Nonrandomized Clinical Studies

One major clinical outcome studied has been childhood malignancy (cancer). No studies have provided convincing evidence that ultrasound exposure increases this risk (Salvesen and Eik-Nes, 1999a).

In 1984, two studies were published examining this outcome. One study retrospectively interviewed the mothers of 1731 children whose deaths from malignancies occurred between 1972 and 1981. The study found no increased risk from ultrasound exposure when compared to 1731 controls (Wilson and Waterhouse, 1984). Similarly, interviews with parents of children diagnosed between 1980 and 1983 revealed the same conclusion (Cartwright et al., 1984).

In 1994, a case–control study found no association between ultrasound exposure and leukemia, lymphoma, or brain tumors (Shu et al., 1994). Another 1994 case–control study investigated the relationship of prenatal ultrasound exposure to astrocytic glioma and primitive neuroectodermal brain tumors diagnosed between 1986 and 1989. In that study, cases had less ultrasound exposure than controls (odds ratio [OR] 0.5, 95% CI 0.3–0.8) (Bunin et al., 1994).

No difference was noted in a case–control study of 1373 children who died of childhood cancer between 1982 and 1984 (Sorahan et al., 1995). A case–control study in 2000 failed to show an increased incidence of lymphatic or myeloid leukemia from ultrasound exposure in children diagnosed between 1973 and 1989 (Naumburg et al., 2000).

In 2002, researchers interviewed parents of children diagnosed with acute lymphoblastic leukemia between 1989 and 1993. No significant difference was noted between ultrasound-exposed cases and nonexposed controls (Shu et al., 2002). A recent case–control study examined ultrasound exposure of children born between 1975 and 1984 who were diagnosed with a brain tumor in childhood. That study found no association (Stalberg et al., 2008).

Short-term clinical outcomes, such as birth weight and Apgar scores (a standard newborn health assessment), have been assessed in multiple studies. In 1978, researchers compared 297 infants exposed to amniocentesis and ultrasound, 661 women who underwent amniocentesis alone, and 949 women exposed to neither. The study found no differences in birth weight less than 2500 g, 5-minute Apgar scores less than eight, overall neurological exam, or 1-year follow-up weight and length.

An increased rate of abnormal grasp and tonic neck reflexes was noted in the amniocentesis/ultrasound group at birth. This difference was not present when the infants were discharged home (Scheidt et al., 1978).

A study population between 1979 and 1980 was randomized to two scans at 18 and 32 weeks or to scans as clinically indicated. No statistical difference was noted in birth weight (Eik-Nes et al., 1984). Between 1979 and 1980, 510 cases were randomized to ultrasound at 19 and 32 weeks. No difference in birth weight, Apgar scores, NICU (neonatal intensive care unit) admissions, or mortality was found when compared to 499 controls (Bakketeig et al., 1984).

No difference in height or weight from birth up to 6 years of age was found among 149 sibling pairs exposed to ultrasound compared to those unexposed between 1975 and 1980 (Lyons et al., 1988). A cohort study published in 1988 reported no overall difference in birth weight. However, the study noted decreasing birth weight with increasing numbers of ultrasounds. Further analysis revealed that the indication for the ultrasound was a possible confounder (an outside factor that could explain the result) (Moore et al., 1988).

In a study published in 1988, 3068 women were randomized between 1985 and 1987 to a screening ultrasound between 13 and 19 weeks, compared to 1279 controls not to receive ultrasound before 19 weeks. Of the control group, only 68.2% never underwent an ultrasound during gestation. Mean Apgar scores were similar. A slightly higher birth weight was noted in the screening group (42 g). However, a difference was noted between spontaneous and nonspontaneous labor. A difference was also noted between those who did and did not report smoking at their first visit (Waldenstrom et al., 1988).

A total of 4691 women were randomized to ultrasound between 16 and 20 weeks, compared with 4619 women for follow-up only. The mean number of ultrasounds performed in the screening group was 2.1, compared to 1.8 for the controls. Birth weight of singletons, 1-minute Apgar scores less than seven, and NICU admissions were similar. A reduction in perinatal mortality (death around the time of birth) for singletons and twins in the screening group was noted. Researchers attributed this reduction to the increased detection of anomalies in the ultrasound group (Saari-Kemppainen et al., 1990).

Randomized Controlled Trials

In addition to short-term parameters, researchers have studied long-term clinical outcomes. In 1984, researchers compared 425 infants exposed to ultrasound between 1968 and 1972 to 381 nonexposed infants. Short-term outcomes including Apgar scores, birth weight, head circumference, and length were similar.

The children were followed up to ages 7–12. No differences were found in long-term outcomes of hearing, vision, and cognitive and behavioral function. However, a nonstatistically significant trend toward dyslexia was seen in the exposed group (Stark et al., 1984).

In two randomized controlled trials, children exposed to ultrasound between 1979 and 1981 were followed up to ages 8–9. No differences were found in vision, hearing, school performance, dyslexia, or speech and neurologic development (Salvesen et al., 1992a, b, 1993, 1994).

This finding was in contrast to a case–control study in 1993 of 72 children. That study reported an increased risk of delayed speech up to 100 months in exposed children (OR 2.8, 95% CI 1.5–5.3) (Campbell et al., 1993).

A randomized trial between 1985 and 1987 followed children up to ages 8–9. That trial noted no difference in growth, vision, hearing, or neurologic development (Kieler et al., 1997, 1998a).

The possibility that prenatal ultrasound exposure could influence handedness was raised by these follow-up trials. Salvesen and colleagues (1993) reported a trend toward nonright-handedness in the exposed group (OR 1.32, 95% CI 1.02–1.71). No difference was later reported when researchers included both males and females. However, an increased risk was found in a subanalysis of males alone (OR 1.33, 95% CI 1.02–1.74) (Kieler et al., 1998b). A meta-analysis of the two studies also suggested an association when analyzing males alone (Salvesen and Eik-Nes, 1999b).

Examining the handedness of adult men yielded conflicting results, depending on the years during which ultrasound exposure occurred (Kieler et al., 2001).

One randomized trial reported an increased rate of intrauterine growth restriction (IUGR, poor fetal growth) in the exposed group. The relative risk for birth weight below the 10th percentile was 1.35 (95% CI 1.09–1.69). The relative risk for birth weight below the third percentile was 1.65 (95% CI 1.09–2.49) (Newnham et al., 1993).

Further analysis of the initial data examined differences in specific measurements of the growth-restricted infants. That analysis noted decreasing bone growth compared to soft tissue development. This finding supports the idea that different tissues have different susceptibilities to ultrasound exposure (Evans et al., 1996).

A follow-up study up to 8 years later, however, found similar physical, neurologic, and developmental parameters in both groups (Newnham et al., 2004).

Long-Term Outcomes

Long-term outcomes have been a major focus of ultrasound safety research. Studies have followed children for years after birth to look for effects on growth, vision, hearing, school performance, speech, and neurologic development.

The results have been largely reassuring. Multiple randomized trials found no differences in vision, hearing, school performance, dyslexia, or speech and neurologic development (Salvesen et al., 1992a, b, 1993, 1994; Kieler et al., 1997, 1998a).

However, some findings deserve attention. One case–control study reported an increased risk of delayed speech up to 100 months in exposed children (OR 2.8, 95% CI 1.5–5.3) (Campbell et al., 1993). Several studies reported a possible link between ultrasound exposure and nonright-handedness, particularly in males (Salvesen et al., 1993; Kieler et al., 1998b; Salvesen and Eik-Nes, 1999b).

One randomized trial reported an increased rate of poor fetal growth in the exposed group (Newnham et al., 1993). A follow-up study 8 years later, however, found similar physical, neurologic, and developmental parameters in both groups (Newnham et al., 2004).

Conclusions

The safety of obstetrical ultrasound is generally supported when ultrasound is used as medically indicated. The medical use of ultrasound by skilled and/or licensed sonographers or physicians is endorsed by the American College of Obstetricians and Gynecologists (ACOG), the AIUM, and the FDA.

The ODS was developed to provide a measure of safety in real time during the exam. This allows the examiner to adjust the length or settings of the exam to maximize safety.

What This Means for Patients

For patients, the main message of this review is reassuring but not absolute. When ultrasound is used for a medical reason and performed by a trained professional, the evidence supports its safety. Decades of studies have looked for links between ultrasound exposure and childhood cancer, birth defects, impaired growth, and developmental problems. Most of these studies found no association.

However, the review also highlights important gaps. Modern ultrasound machines can produce much higher energy levels than the machines used in most of the older safety studies. The safety of these newer machines has not been confirmed by clinical studies. The same is true for Doppler ultrasound, 3-D ultrasound, and 4-D ultrasound.

This means that patients should be cautious about non-medical ultrasound, such as keepsake imaging. The review specifically raises concerns about the safety of these practices.

Another key point is that the person performing the ultrasound is responsible for monitoring safety during the exam. Surveys have found that many ultrasound practitioners have poor knowledge about ultrasound safety in pregnancy. This finding suggests that patients may want to ask their provider about the safety measures being used.

Limitations of the Evidence

The evidence reviewed here has several limitations. First, most clinical studies were performed with older machines that had much lower output potential than modern machines. This means the safety profile of current equipment is not fully established.

Second, many of the findings about thermal and mechanical effects come from laboratory and animal studies, not from human studies. The application of animal findings to human fetuses is unclear. For example, fetal lungs contain amniotic fluid instead of air, so findings about lung hemorrhage in animals may not apply to human fetuses.

Third, some human studies produced conflicting results. For example, some studies reported a possible link between ultrasound exposure and nonright-handedness, particularly in males, while others did not. One study reported an increased risk of delayed speech, while randomized trials found no difference in speech development.

Fourth, some studies had possible confounders. For example, one study found decreasing birth weight with increasing numbers of ultrasounds, but further analysis revealed that the indication for the ultrasound was a possible confounder.

Fifth, the long-term safety profile of Doppler ultrasound, 3-D ultrasound, and 4-D ultrasound is unknown. These modes are being used more frequently, but they have not been studied as thoroughly as standard B-mode imaging.

Recommendations

Based on this review, patients and providers can consider the following recommendations:

1. Use ultrasound when it is medically indicated. The evidence supports the safety of ultrasound for medical purposes when performed by trained professionals.
2. Be cautious about non-medical ultrasound, including keepsake imaging. The safety of these practices has not been established.
3. Ask your provider about the safety measures being used during your ultrasound exam. The output display standard (ODS) provides real-time information about thermal and mechanical indices.
4. Understand that the person performing the ultrasound is responsible for monitoring safety. Ask whether your provider is trained and knowledgeable about ultrasound safety.
5. Recognize that Doppler, 3-D, and 4-D ultrasound have not been confirmed as safe in long-term studies. Discuss the reasons for using these modes with your provider.
6. Keep in mind that the thermal index (TI) and mechanical index (MI) are displayed on modern machines. A value less than one is generally accepted as safe.

Frequently Asked Questions

Is ultrasound safe for my unborn baby?

When ultrasound is used for a medical reason and performed by a trained professional, the evidence supports its safety. Decades of studies looked for links to childhood cancer, birth defects, impaired growth, and developmental problems, and most found no association. However, the safety of newer, more powerful machines and of Doppler, 3-D, and 4-D modes has not been confirmed.

What are the thermal and mechanical risks of ultrasound?

Scientists propose two possible mechanisms of harm: heating of tissues (thermal effects) and mechanical forces such as cavitation. The machine displays a thermal index (TI) and mechanical index (MI) to estimate these risks. A value less than one is generally accepted as safe. The person performing the exam is responsible for monitoring these indices.

What does a thermal index (TI) or mechanical index (MI) less than one mean?

The thermal index (TI) is a unitless approximation of heating risk, and the mechanical index (MI) approximates mechanical strain. A value less than one is generally accepted as safe. Modern machines display these indices in real time, allowing the examiner to adjust settings or exam length to maximize safety. You can ask your provider about the values being used.

Are keepsake 3-D or 4-D ultrasounds safe?

The safety of 3-D and 4-D ultrasound, including keepsake imaging requested for non-medical reasons, has not been proven. These practices have not been confirmed as safe in long-term studies. The review specifically raises concerns about non-medical ultrasound. Patients should be cautious about keepsake imaging and discuss any use of these modes with their provider.

Is Doppler ultrasound safe during pregnancy?

Doppler ultrasound can reach significantly higher output levels than standard B-mode imaging. Its long-term safety profile is unknown. Doppler is being used more often in pregnancy, including middle cerebral artery Doppler. Because of higher energy and bone absorption, the safety of Doppler has not been confirmed by clinical studies. Discuss the reasons for using Doppler with your provider.

Could ultrasound cause birth defects or childhood cancer?

Most clinical studies found no link between ultrasound exposure and childhood cancer, birth defects, or impaired development. For example, multiple case-control studies found no association with leukemia, lymphoma, or brain tumors. However, a few studies raised questions about birth weight, left-handedness, and speech delay. The evidence is largely reassuring but not absolute.

What should I ask my provider about ultrasound safety?

Ask about the safety measures being used during your exam. The output display standard (ODS) provides real-time thermal and mechanical indices. Ask whether your provider is trained and knowledgeable about ultrasound safety, because surveys found many practitioners have poor knowledge. Also discuss the reasons for using Doppler, 3-D, or 4-D modes, since their long-term safety is unknown.

Should I get a second opinion before having Doppler, 3-D, or 4-D ultrasound during pregnancy?

A second opinion can help when Doppler, 3-D, or 4-D imaging is proposed, since their long-term safety is not confirmed and they reach higher output levels than standard B-mode imaging. Keepsake imaging for non-medical reasons has not been proven safe. The person performing the exam monitors safety, and surveys found many practitioners have poor knowledge of ultrasound safety in pregnancy. A second opinion can clarify whether a mode is medically indicated and what thermal and mechanical index values apply. Diagnostic Detectives Network provides independent expert second opinions.

Source Information

Original article title: Prenatal Diagnosis - 2009 - Houston - The safety of obstetrical ultrasound a review

Authors: Laura E. Houston, Anthony O. Odibo, and George A. Macones

Author affiliations: Department of Obstetrics and Gynecology, Washington University in St. Louis, St. Louis, MO, USA; Ultrasound & Genetics, Division of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, Washington University in St. Louis, St. Louis, MO, USA

Publication: Prenatal Diagnosis, 2009; 29: 1204–1212. Published online 6 November 2009 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/pd.2392

Article type: Review

Dates: Received 18 February 2009; Revised 3 September 2009; Accepted 4 September 2009; Published online 6 November 2009

Copyright: 2009 John Wiley & Sons, Ltd.

This patient-friendly article is based on peer-reviewed research.