Is Ultrasound Safe?
Reproduced
with permission from the newsletter of Sarah J Buckley, August 2009, and
adapted from "Ultrasound: Scans: Cause
for Concern" Chapter 5 in Gentle Birth, Gentle Mothering
(2009). by Sarah J Buckley MD See this chapter for full
information and references.
For
more of Sarah’s writing, and to subscribe to her newsletter, see www.sarahjbuckley.com
Ultrasound has been used to examine the unborn baby for more than forty years, but very little research has been done to determine possible short- or long-term effects on offspring.
Ultrasound is known to affect living tissues in several ways. Firstly, it causes heating in tissues while they are targeted by the sonar waves emitted by the transducer- the part of the machine that is placed on the pregnant woman's belly. Heating is especially likely later in pregnancy, when the baby's bones calcify: harder bones can reflect and concentrate heat more effectively. Researchers are especially concerned about heating effects on the growing (unborn or newborn) brain, which is especially vulnerable to heat, a known neurotoxin, because of proximity to the skull bones.(Barnett 2001)
The force of the ultrasound waves also causes the fluid within tissues to move, an effect known as "acoustic streaming". The velocity that is created can disrupt cell membranes and may explain observed effects such as increased stickiness in red cells following ultrasound exposure(Pohl, Rosenfeld et al. 1995), and the effects on brain cell migration mentioned below.
Ultrasound waves can also cause “cavitation”, an effect that produces extreme temperatures and free radical formation in the small bubbles of air that exist in some mammalian tissues. This effect is significant for the newborn gut and lung, which are filled with air, but its significance for the unborn is not known.(Church and Miller 2007)
Studies of animals exposed to ultrasound in the womb have found effects including low birth weight;(Tarantal, Gargosky et al. 1995) low numbers of white (immune) blood cells in infancy;(Tarantal, Gargosky et al. 1995) damage to myelin nerve sheaths;(Ellisman, Palmer et al. 1987) brain hemorrhage;(Dalecki, Child et al. 1999) and locomotor and learning difficulties in adulthood.(Suresh, Uma Devi et al. 2002)
A recent study in rats found abnormalities in neuronal migration (the process by which brain cells travel to the appropriate part of the brain during development) which correlated with duration of exposure to ultrasound in the womb. In this study, no rats with over 600 minutes total exposure to ultrasound during their 22 day gestation survived to 10 days.(Ang, Gluncic et al. 2006) Problems with neuronal migration have been linked with autism and dyslexia in humans
Human studies are very limited. Randomized trials from thirty years ago found no effects on learning, hearing or speech but found a higher risk of non-right-handedness associated with ultrasound exposure.(Salvesen, Bakketeig et al. 1992; Salvesen, Vatten et al. 1992; Salvesen, Jacobsen et al. 1993; Salvesen, Vatten et al. 1994) These findings have been replicated in other observational studies.(Kieler, Cnattingius et al. 2001; Kieler, Cnattingius et al. 2002) This effect implies that ultrasound can change the lateralization of the brain, which represents a a significant shift in brain development. Note that at the time of these studies, machine output was much lower, and examinations much shorter than today.
The only other large randomized trial with long-term follow-up involved offspring randomized to regular Doppler examination (see below) vs. standard ultrasound. Those exposed to Doppler were more likely to be born with low birth weight- the condition that Doppler was supposed to detect.(Newnham, Evans et al. 1993) Low birth weight has also been found in animal studies and may reflect interference with the growth hormone system. Comparing the two groups at age 8, researchers found no difference in learning or motor function,(Newnham, Doherty et al. 2004) but note that there was no unexposed (“control”) group, so the true effects of ultrasound vs. no exposure cannot be determined from this study.
Two other randomized Doppler studies have also had concerning results. In one study, twice as many exposed babies died around the time of birth (Davies, Gallivan et al. 1992), and another found worse condition in labour and at birth (more fetal distress and lower APGAR) among babies randomized to intensive Doppler examinations in pregnancy.(Newnham, O'Dea et al. 1991)
Doppler is used to measure flow in blood vessels and is also used, in low exposures, to detect and record the fetal heart, eg in hand-held machines and with fetal heart-rate monitoring. Doppler uses continuous waves, which give higher exposure than normal ultrasound, which uses pulsed waves.
This is very concerning because, in the last two decades, machines have become more sophisticated and output significantly higher. In 1993, the US FDA approved the sale of ultrasound machines with 8x higher output that previously allowed.
Exposure of unborn babies to these higher outputs has not been tested for long-or even short-term safety. Researchers note “There are no epidemiological [large population] prenatal ultrasound studies with commercially available ultrasound devices produced after 1990.”(Salvesen 2007) This is despite increasing awareness by, for example, the American Institute of Ultrasound in Medicine, who stated in 2000 “ ...the responsibility of an informed decision concerning possible adverse effects of ultrasound in comparison to desired information will probably become more important over the next few years.”(Fowlkes and Holland 2000)
As authors of a large review of ultrasound safety in the prestigious
Note that this information does not “prove” that ultrasound is unsafe, or that a single exposure during pregnancy will have long-term effects on your baby. Rather it raises concerns, and suggests that the possible risks of ultrasound must be considered by pregnant women and their carers when making decisions about its use.
Ultrasound is a medical intervention that requires the same large body of high-quality research to establish safety, short and long-term, as any other intervention involving the vulnerable unborn baby.At this time, it is impossible to say whether the current use of prenatal ultrasound is safe or unsafe, because this research has not been done.
Selected references
Ang, E. S., Jr., V. Gluncic, et al. (2006). "Prenatal exposure to ultrasound waves impacts neuronal migration in mice." Proc Natl Acad Sci U S A 103(34): 12903-10.
Barnett, S. B. (2001). "Intracranial temperature elevation from diagnostic ultrasound." Ultrasound Med Biol 27(7): 883-8.
Church, C. C. and M. W. Miller (2007). "Quantification of risk from fetal exposure to diagnostic ultrasound." Prog Biophys Mol Biol 93(1-3): 331-53.
Dalecki, D., S. Z. Child, et al. (1999). "Hemorrhage in murine fetuses exposed to pulsed ultrasound." Ultrasound Med Biol 25(7): 1139-44.
Davies, J. A.,
Ellisman, M. H., D. E. Palmer, et al. (1987). "Diagnostic levels of ultrasound may disrupt myelination." Exp Neurol 98(1): 78-92.
Fowlkes, J. B. and C. K. Holland (2000). "Mechanical bioeffects from diagnostic ultrasound: AIUM consensus statements. American Institute of Ultrasound in Medicine." J Ultrasound Med 19(2): 69-72, p 70.
Kieler, H., S. Cnattingius, et al. (2001). "Sinistrality--a side-effect of prenatal sonography: a comparative study of young men." Epidemiology 12(6): 618-23.
Kieler, H.,
Marinac-Dabic, D., C. J. Krulewitch, et al. (2002). "The safety of prenatal ultrasound exposure in human studies." Epidemiology 13(3 Suppl): S19-22., p S19.
Newnham, J. P., D. A. Doherty, et al. (2004). "Effects of repeated prenatal ultrasound examinations on childhood outcome up to 8 years of age: follow-up of a randomised controlled trial." Lancet 364(9450): 2038-44.
Newnham, J. P., S. F. Evans, et al. (1993). "Effects of frequent ultrasound during pregnancy: a randomised controlled trial." Lancet 342(8876): 887-91.
Newnham, J. P., M. R. O'Dea, et al. (1991). "Doppler flow velocity waveform analysis in high risk pregnancies: a randomized controlled trial." Br J Obstet Gynaecol 98(10): 956-63.
Pohl, E. E., E. H. Rosenfeld, et al. (1995). "Effects of ultrasound on agglutination and aggregation of human erythrocytes in vitro." Ultrasound Med Biol 21(5): 711-9.
Salvesen, K. A. (2007). "Epidemiological prenatal ultrasound studies." Prog Biophys Mol Biol 93(1-3): 295-300, p 301.
Salvesen, K. A., L. S. Bakketeig, et al. (1992). "Routine ultrasonography in utero and school performance at age 8-9 years." Lancet 339(8785): 85-9.
Salvesen, K. A., G. Jacobsen, et al. (1993). "Routine ultrasonography in utero and subsequent growth during childhood." Ultrasound Obstet Gynecol 3(1): 6-10.
Salvesen, K. A., L. J. Vatten, et al. (1994). "Routine ultrasonography in utero and speech development." Ultrasound Obstet Gynecol 4(2): 101-3.
Salvesen, K. A., L. J. Vatten, et al. (1992).
"Routine ultrasonography in utero and subsequent vision and hearing at
primary school age." Ultrasound Obstet Gynecol 2(4): 243-4, 245-7.
Suresh, R., P. Uma Devi, et al. (2002). "Long-term effects of diagnostic ultrasound during fetal period on postnatal development and adult behavior of mouse." Life Sci 71(3): 339-50.
Tarantal, A. F., S. E. Gargosky, et al. (1995). "Hematologic and growth-related effects of frequent prenatal ultrasound exposure in the long-tailed macaque (Macaca fascicularis)." Ultrasound Med Biol 21(8): 1073-81.
See Gentle Birth, Gentle Mothering chapter 5 for more information and full references