When Do The Benefits Of Ultrasound Outweigh The Dangers?

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When Do The Benefits Of Ultrasound Outweigh Dangers?

Summary: To date, bioeffects studies in humans do not substantiate a causal relationship between diagnostic ultrasound exposure during pregnancy and adverse biological effects to the fetus. However, all epidemiologic studies were conducted with commercially available devices predating 1992, having outputs not exceeding 94 mW/cm2, whereas since 2002 obstetric ultrasound devices are allowed by the FDA to reach 720 mW/ cm2. Available evidence is insufficient to conclude that there is a causal relationship between obstetric diagnostic ultrasound exposure and adverse effects to the fetus. However, all agree there is a need for further investigation of potentially adverse developmental effects. Ultrasound produces heat in the tissues it irradiates. The fetus before 10 weeks has no blood circulation and therefore is unable to rid itself easily of the thermal effects of ultrasound until after 10 weeks gestation. The FDA in 2002, the SOGC in 2005, The American Academy of Family Practitioners in 2009 and ACOG in 2009  all agree that because of the potential for 'adverse effects’, prenatal ultrasound should not be offered or used routinely.  It is revealing that the term adverse effects is always used in favor of "dangers" of obstetric ultrasound.  A search for "Danger obstetric ultrasound" comes up with 0 hits on Medline whereas "Danger radiation" produces over 400."

Risk-benefit issues are extremely important in clinical practice. In situations where risks of adverse effects appear so low and clinical benefits are great,  there is no justification to withhold the use of ultrasound.

In the following cases, the benefits outweigh the possible risks:

1) To Turn Breech:  If the baby is breech, ultrasound dramatically improves outcomes of external version at 37 weeks.  When attempting to turn breech fetus without ultrasound, every once in a while a head down baby is turned to breech by accident.

2. Locating Placenta Before vbacs: To confirm placenta is not located in the scar, at term, prior to attempting vaginal birth after cesarean.  Fifty years ago, placenta accreta was rare, occurring 1 in 3,300 births.(Esh-Broder 2011) .   Today, the rates are much higher.  The highest reported rates are for IVF births for which the reported rate of placenta accreta in 1/60 (Esh-Broder 2011).  After one cesarean, the reported rate of placenta accreta is  between 1 in 150 to 1 in 300.(Marshall 2011)  If the placenta is deeply imbedded in the scar, then during repeat CS, the surgeons can attempt to save the uterus, or at least save the woman before she bleeds to death.  Placenta accreta is a known risk for hysterectomy but the actual rate of hysterectomy in the presence of placenta accreta has never been reported.

3. IVF

4. Rare occurrences:

  • Evaluation of pelvic tumors or fibroids
  • Evaluation of hydatidiform mole
  • Evaluation for ectopic pregnancy
  • Adjunct to fetal transfusion and amniocentesis
  • Confirmation of fetal death
  • Confirmation of presentation where practitioners are not taught  to evaluate presentation without ultrasound

In all other situations, the possible risk of using ultrasound is not outweighed by suspicion of possible benefit. The FDA in 2002(1), the SOGC in 2005(2), The American Academy of Family Practitioners in 2009(3) and ACOG in 2009 (4) all agree that because of the potential for 'adverse effects’, prenatal ultrasound should not be offered or used routinely.

Evaluating dangers of ultrasound

The best way to understand what ultrasound is capable of, search: Yoav Medan Ultrasound surgery heal without cuts, and watch the video.   In it, Yoav Medan mentions how  ultrasound breaks up kidney stones.  He uses magnetic resonance imaging to locate a target in the body and then focused ultrasound to destroy uterine fibroids and  kill small parts of the brain that are causing tremors, and defects in the spine that cause back pain. 

Low-intensity pulsed ultrasound is used clinically to accelerate the bone fracture repair process and induce healing in humans. Low-intensity pulsed ultrasound also has been shown to enhance repair of soft tissue damage and accelerate nerve regeneration in animal models. Although such exposures to low intensity do not appear to cause damage to exposed tissues, they do raise questions about the threshold that might induce potentially adverse developmental effects in the fetus. It is stated to be plausible (Stratmeyer 2008) that there is no association between use of diagnostic ultrasound during pregnancy using commercial devices available before 1990 and

1. Childhood cancer;

2. Reduced birth weight;

3. Impaired childhood growth; and

4. Neurologic development in childhood.

The epidemiologic evidence showing ultrasound to be safe during pregnancy is based on exposure conditions prior to 1992. There are no epidemiologic studies that have tested the safety of prenatal ultrasound using high levels since 1992.   In 1992, the FDA increased the acoustic limits of ultrasound machines from 94 to 720 mW/cm2 for fetal applications, without demanding further studies.  The safety of new Doppler imagining (spectral and color) technologies, 3-dimensional imaging, and the use of ultrasound contrast agents as well as that of probe self-heating has not been investigated.  Today, it is impossible to find a large matched control group to establish a baseline for outcomes without ultrasound.

Major anomalies occur in 3% to 5% of the general human population.

Epidemiologic evidence may have difficulty establishing a causal relationship between ultrasound and adverse effects because of the large number of anomalies that occur naturally. In any case, epidemiology does not determine causality. Causality is judged by the strength of the association, consistency, specificity, the relationship in time, biological plausibility, and experimental evidence.  Low birth weight(1993), delayed speech(1993), dyslexia(1984), and non–right-handedness(1998) have been associated with ultrasound in these four epidemiological studies but further studies were unable to repeat these findings.  (Abramowicz 2008)

Biological damage has been shown using some forms of ultrasound in animals and in vitro cells. Subtle or transient effects in humans are possible, but none have been consistently demonstrated so far.

The two most likely mechanisms for bioeffects on the fetus are heating of cells and cavitation.  It is known that the fetus is already about a degree hotter than its mother, and has no way to easily get rid of excess heat.  Ultrasound raises the temperature of whatever tissue it bounces off from. Thermally induced teratogenesis i.e. defects caused by raising temperature, has been shown in many animal studies, as well as several controlled human studies. Conditions present in early pregnancy, such as lack of blood perfusion, may favor bioeffects. The embryo has no blood circulation until about 10 weeks, therefore, the fetus before 10 weeks has not way to easily rid itself of heat by carrying it away. 

At about weeks 4 to 5, the gestational sac is about the size of the eye (2.5 cm in diameter), and by week 8, it is around 8 cm in diameter. There is no perfusion in very early gestation. Only at about weeks 10 to 11 does the embryonic circulation actually linkup with the maternal circulation. At that stage, in addition to blood flow removing excess heat,  there is a switch from previously hypoxic conditions to normo-oxic conditions and radical scavengers  like superoxide dismutase begin to remove free radicals. 

Technical terms: Thermal effect (TI) is when the temperature is raised 1 degree or more.  Mechanical effects (MI) are defined as when the temperature is raised less than 1 degree. Mechanical effects (MI) also includes cavitation which means mechanical or chemical reactions that happen because oxygen or air is hit by sound waves.  It also refers to  sonoluminescence, which means the emission of short bursts of light from bubbles imploding in a liquid excited by sound.

In a study in which 200 questionnaires were answered by physicians, sonographers, and midwives, all using Doppler ultrasound on a daily or weekly basis,  only 22% could explain what thermal index (TI) means and only 11% could give a correct explanation for the mechanical index (MI). Only 28% correctly indicated where, on their own machines, they could find the information about the amount of ultrasound the fetus is being exposed to.  (Sheiner 2008)

While ultrasound may have bioeffects, based mainly on two mechanisms of action, thermal effects  and mechanical effects, many uncertainties persist. These include, among others,

Is there a minimum amount of ultrasound microwaves strength that can cause bioeffects? If so, what is the threshold?

What is the effect of exposure duration?

What is the effect of the type of exam- belly or vaginal?

What is the effect of the examiner’s skills?

What is the effect of  multiple examiners?

What is the influence of cumulative doses, repeated scans and at what intervals?

What is the biological response of fetuses at different gestational ages? or brain versus bones? or elevated temperature in the mother or medical conditions complicating the pregnancy?

False Positives: The ability of ultrasound to detect anomalies in the second trimester varies from 13% to 85%(4) depending on abilities of the technician, and involves high rates of false positives. Most women get ultrasounds to be reassured but 10% of second trimester ultrasounds detect one or more false positive markers for Downs syndrome and 10% detect other markers or out of range measurements.(1)  None of these women will be reassured although most of their babies are perfect. Ultrasound can accurately diagnosis 91% of fetuses affected by Downs syndrome but will false positively diagnose 5% of normal fetuses as having Downs syndrome.(1)  

Only 3 severe defects have been treated successfully in utero: Twin twin transfusion syndrome, Bladder obstructions  and Sacrococcygeal tumor. Babies diagnosed with these severe defects usually have other defects as well, and survival rates are less than 50% even with early diagnosis and heroic, not to mention costly prenatal interventions. Since the ability of ultrasound to both diagnose and improve outcomes of severe defects is close to non-existant in most locations, routine prenatal ultrasound screening is most often simply the best way to terrify a pregnant woman. 

[For the first-hand peer-reviewed research on the potential, unintended adverse effects of Obstetric Ultrasonography view our research page on the topic, which now includes 26 studies]


1.Marinac-Dabic D, Krulewitch CJ, Moore RM Jr. Center for Devices and Radiological Health, Food and Drug Administration. The safety of prenatal ultrasound exposure in human studies. Epidemiology. 2002 May;13(3 Suppl):S19-22. 

2.Bly S, Van den Hof MC; Diagnostic Imaging Committee, Society of Obstetricians and Gynaecologists of Canada. Obstetric ultrasound biological effects and safety. J Obstet Gynaecol Can. 2005 Jun;27(6):572-80. 

3. Ultrasonography, diagnostic in Ob/Gyn (PositionPaper). [Accessed 2 Jan 2010]. 

4. ACOG Practice Bulletin No. 101: Ultrasonography in Pregnancy. (2009).OBGYN 113(2):451-61.Esh-Broder E, Ariel I, Abas-Bashir N, Bdolah Y, Celnikier DH. Placenta accreta is associated with IVF pregnancies: a retrospective chart review. BJOG. 2011;118(9):1084-9.

Marshall NE, Fu R, Guise JM. Impact of multiple cesarean deliveries on maternal morbidity: a systematic review. Am J Obstet Gynecol. 2011;205(3):262.e1-8.

Stratmeyer ME, Greenleaf JF, Dalecki D, Salvesen KA. Fetal ultrasound: mechanical effects. REVIEW. J Ultrasound Med. 200;27(4):597-605.

Abramowicz JS, Barnett SB, Duck FA, Edmonds PD, Hynynen KH, Ziskin MC. Fetal thermal effects of diagnostic ultrasound. J Ultrasound Med. 2008 Apr;27(4):541-59; quiz 560-3.

Abramowicz JS, Fowlkes JB, Skelly AC, Stratmeyer ME, Ziskin MC. Conclusions regarding epidemiology for obstetric ultrasound. J Ultrasound Med. 2008;27(4):637-44.

Sheiner E, Abramowicz JS. Clinical end users worldwide show poor knowledge regarding safety issues of ultrasound during pregnancy. J Ultrasound Med. 2008 Apr;27(4):499-501.

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