Ultrasound Safety
By Sarah Buckley, MD
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).
See this chapter for full information and references.
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 US
journal Epidemiology (2002) concluded, "Until long-term effects can be
evaluated across generations, caution should be exercised when using
this modality during pregnancy."(Marinac-Dabic, Krulewitch et al. 2002)
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 you need to
consider the possible risks of ultrasound when making decisions about
ultrasound for your baby.
You may also consider that the best medical evidence shows no benefit
to mother or baby from a routine ultrasound in early pregnancy, apart
from an increased number of terminations of pregnancy due to
abnormalities detected on ultrasound. (Nelison 2000) (See my book for
more about the accuracy of ultrasound in detecting abnormalities)
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., S. Gallivan, et al. (1992). "Randomised controlled trial
of Doppler ultrasound screening of placental perfusion during
pregnancy." Lancet 340(8831): 1299-303.
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., S. Cnattingius, et al. (2002). "First trimester ultrasound scans and left-handedness." Epidemiology 13(3): 370.
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.
Neilson, J. P. (2000). "Ultrasound for fetal assessment in early pregnancy." Cochrane Database Syst Rev(2): CD000182.
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.
More Ultrasound Safety Articles, Links & Resources: