FROM: Buckley S. Gentle Birth, Gentle Mothering. Brisbane, Australia: One Moon Press; 2005 (99-101).
Ultrasound waves are known to affect tissues in two main ways. First, the sonar beam causes heating of the highlighted area by about one degree Celsius (1.8F). This is presumed to be non-significant based on whole-body heating in pregnancy, which seems to be safe up to 2.5C (4.5F). Doppler scans, which use continuous waves, can cause more significant heating, especially in the baby’s developing brain.
The second recognized effect is cavitation, where the small pockets of gas that exist within mammalian tissue vibrate and then collapse. In this situation, temperatures of many thousands of degrees Celsius in the gas create a wide range of chemical products, some of which are potentially toxic. These violent processes may be produced by micro-second pulses of the kind which are used in medical diagnosis. The significance of cavitation effects in human tissue remains uncertain.
However, a number of studies have suggested that these effects may be of real concern in living tissues. The first study suggesting problems was a study on cells grown in the lab. Cell abnormalities caused by exposure to ultrasound were seen to persist for several generations. A more recent study involving newborn rats, who are at a similar stage of brain development to humans at four to five months in utero, showed that ultrasound can damage the myelin that covers nerves, indicating that the nervous system may be particularly susceptible to damage from this technology.
Another animal study published in 2001 showed that exposing mice to dosages typical of obstetric ultrasound caused a 22 percent reduction in the rate of cell division, and a doubling of the rate of apoptosis (programmed cell death) in the cells of the small intestine. Other researchers have found that a single ten-minute ultrasound exposure in pregnancy affects the locomotor and learning abilities of mice offspring in adulthood, with a greater effect from longer exposure time.
Experts in this area have expressed concern, especially in relation to exposure of the developing central nervous system, whose tissues are sensitive to damage by physical agents such as heat and ultrasound. S.B. Barnett notes that heating of the baby’s brain is more likely after the first trimester (three months) as the baby’s bone is more developed, and can reflect and concentrate the ultrasound waves. S.B. Barnett warns, “When modern sophisticated equipment is used at maximum operating settings for Doppler examinations, the acoustic outputs are sufficient to produce obvious biological effects.”
R. Mole comments: “If exposure to ultrasound… does cause death of cells, then the practice of ultrasonic imaging at 16-18 weeks will cause loss of neurons [brain cells] with little prospect of replacement of lost cells… The vulnerability is not for malformation but for mal-development leading to mental impairment caused by overall reduction in the number of functioning neurons in the future cerebral hemispheres.”
Recent research has found that ultrasound can induce bleeding in the lung. The American Institute of Ultrasound in Medicine (AIUM) recently concluded, “There exists abundant peer-reviewed published scientific research that clearly and convincingly documents that ultrasound at commercial diagnostic levels can produce lung damage and focal hemorrhage [bleeding] in a variety of mammalian species… The degree to which this is a clinically significant problem in humans is not known.”
Studies on humans exposed to ultrasound have shown that possible adverse effects include premature ovulation, miscarriage or preterm labor, low birth weight, poorer condition at birth, perinatal death, dyslexia, delayed speech development, and less right-handedness. Non-right handedness (left-handedness and ambidexterity) is a consistent finding in many studies and is, in other circumstances, seen as a marker of damage to the developing brain. One Australian study showed that babies exposed to five or more Doppler ultrasounds were 30 percent more likely to develop intrauterine growth retardation (IUGR) – a condition that ultrasound is often used to detect.
A recent summary of the safety of ultrasound in human studies, published in May 2002 in the prestigious US journal Epidemiology suggested: “Continued research is needed to evaluate the potential adverse effects of ultrasound exposure during pregnancy. These studies should measure the acoustic output, exposure time, number of exposures per subject, and the timing during the pregnancy when exposure(s) occurred.” These authors concluded: “Until long-term effects can be evaluated across generations, caution should be exercised when using this modality during pregnancy.”