A 1997 study found that significant temperature increases can occur at or near to bone in the fetus starting in the second trimester, if the beam is held stationary for more than 30 seconds in some pulsed Doppler applications.
This in turn can lead to heating of sensory organs incased in bone.
Doppler ultrasound, which uses continuous rather than pulsed waves, has been shown to cause significant heating – especially in the baby’s developing brain.
A recent study suggests that heating in late-pregnancy fetal tissues exposed to normal pulsed and continuous Doppler ultrasound may be higher than what is regarded as safe: 2.5 to 10.4 degrees F (1.4 – 5.8 C) respectively.
An Australian study found babies that received more than 5 Dopplers were 30% more likely than babies that received routine (pulsed) ultrasound to develop intrauterine growth retardation (IUGR).
Their findings suggested that a child with delayed speech was twice as likely to have been exposed to prenatal ultrasound.
(Note that this is a correlation and doesn’t prove causation.)On the other hand, a recent World Health Organization (WHO) review of the literature in 2009 concluded that “exposure to diagnostic ultrasonography appears to be safe.”However, even in this review they did express some concern about the association between left-handedness in males and exposure to Doppler ultrasound.
Another study found exposing adult mice to dosages typical of obstetric ultrasound caused a 22 percent reduction in rate of cell division and a doubling of the rate of apoptosis of cells in small intestine.
Other research has found that ultrasound induces bleeding in the lungs among other mammals, including newborns and young animals.