I had a busy weekend teaching, and wanting to do a little more research on prenatal ultrasound… so I am a few days behind. But let’s begin.
During his 30-year career, Dr. Christopher Olson observes that the use of ultrasound has “ballooned.” Initially doctors were warned about potential risks to the developing fetus from the heat generated by ultrasound waves, so diagnostic ultrasound was used with caution for specific purposes.
Ultrasound has become the standard of care for diagnosis in medicine and is even used in physical therapy to heat-treat deep muscles. But, Olson observes, “amniotic fluid absorbs more heat energy than solid tissue. And, the developing brain is known to be sensitive to a number of “environmental stressors” such as alcohol, drugs, and hot tubs. He raises the question, “Is it possible that the amount of heat generated by multiple ultrasounds lowers the threshold in a fetus that puts it at increased genetic risk of autism and related neurological disease?”1
Ultrasound has become as popular as apple pie, but some of you may not actually know how it works and how it affects fetal tissues. So allow me to present or review the basics.
Ultrasound is a pressure wave that causes air around it to vibrate. It works like radar, sending sound waves to a “target” and making a “picture” from the sound waves that bounce back. In prenatal ultrasound the “target” is the baby’s yet undeveloped, and developing, body, and/or the developing placenta or amniotic fluid.
Since the expectant mother cannot feel (or hear) the high frequency sound waves or rise in temperature, she is not aware or alarmed by what is happening to her baby. However, when receiving ultrasound waves, babies tend to move away from the stream of high-frequency sound waves. They may be feeling vibrations, heat, or both.
A study was conducted to “hear” what babies hear when they are subjected to ultrasound. Ultrasound waves, aimed directly at a miniature microphone placed in a woman's uterus, recorded sound "as loud as a subway train coming into the station."(Samuel, 2001)2
Ultrasound waves heat the baby’s tissues when “taking a picture” or hearing fetal heart tones. The heat waves from ultrasound or doppler may be beneficial in treating deep muscle spasm in physical therapy, but it may present subtle or long term effects in the developing fetus. Although the research is still “inconclusive” before consenting to diagnostic ultrasound or getting a “keepsake” ultrasound picture at the mall, mothers and fathers need to understand why a temperature increase can cause significant damage to a developing fetus's central nervous system.(Miller, 2002)3
Why Heating the Intrauterine Environment and Baby May Be Harmful
Pregnant women are advised to stay out of hot tubs, hot baths, steam rooms, and saunas. Here’s why: The mother’s core temperature is 98.6. When she is immersed in a hot tub or bath, her body cannot cool off via perspiration, thus raising her core temperature by as much as 2 degrees Fahrenheit.
When a mother’s core temperature is elevated of as little as 2 degrees Fahrenheit, it can result in embryonic death, miscarriage, slowing down of fetal growth and weight gain, and developmental defects. (Edwards, M.J. et.al, 2003)4 One study reported that "women who used hot tubs or saunas during early pregnancy face up to triple the risk of bearing babies with spina bifida or brain defects."(Milunsky, A., et al. 1992)5
The unborn baby is in a little hot tub of its own in the mother’s body. So if the mother’s core temperature goes up —- for any reason (maternal fever, hot tubs, diagnostic ultrasound or keepsake ultrasound) -- her baby cannot escape elevated temperatures in the womb. This is important because, across mammalian species, elevated maternal or fetal body temperatures have been shown to result in birth defects in offspring. (Miller, M.W., et al. 2002)6
What’s interesting here is that mothers are warned about hot tubs, but not about ultrasound heat. Even if the research is still "inconclusive," mothers and fathers deserve informed consent, they deserve to be warned about potential risk of ultrasounds —- so they can make an informed decision about whether or not to take this risk for their baby.
Ultrasound heats bone at a different rate than muscle, soft tissue or amniotic fluid.7 As the baby grows in utero and its bones calcify, they absorb and retain more heat from ultrasound. During the third trimester, the baby's calcified skull can heat up 50 times faster than its surrounding tissue (Barnett, 1998), subjecting parts of the enclosed brain to secondary heat that can continue after the ultrasound exam has concluded.8
“The amount of ultrasound-induced [brain] heating increases with gestational age and the development of fetal bone. The rate of heating near bone is rapid; 75% of the maximum heating occurs within 30 seconds.(Barnett, 2001)9
There are more questions than answers at this time. One thing we have to consider is that we are trusting the health and well-being of our next generation to a technology that has not yet been proven safe, even though it has become the standard of care. More patience, caution, and research is needed to investigate:
• Acoustic output (more about this topic in the next blog entry)
• The amount of time the fetus is exposed to ultrasound waves
• If, and in what way, the number of ultrasounds during pregnancy affect the baby
• What, if any, teratogenic/toxic effects result at different stages of fetal development
But this research will not easy to do; “there are roadblocks to this research,” Olson warns, namely “fear-based resistance among clinicians in a litigious society to give up ‘safety’ of the current standard of care.”10 And, with the majority of mothers and babies now exposed to routine prenatal ultrasound, it will be more difficult to find control groups for comparative study.
In the meantime, it is up to us to inform parents, and for parents to ask themselves if the risks outweigh the benefits.
Parents need to know that the frequent use of ultrasound has not been shown to improve baby’s health or birth outcome. Except for diagnosing the gender of the baby, skilled birth attendants can make the other assessments with their hands. But this skill is rapidly being lost as more doctors and midwives refer parents to ultrasound rather than take the time to do a thorough hands-on exam.
We will continue with a review of the literature and other factors to consider in a day where ultrasounding our next generation in utero has become the almost unquestioned standard of care.
1 Olson, C.D. (2009). Does ultrasound increase risk for autism? Journal of American Osteopath Association. 109(2), 71-72. Retrieved from Pub Med: http://www.Jaoa.org/cgi/reprint/109/2/71
2 Samuel, Eugenie. 2001. Fetuses can hear ultrasound examinations. New Scientist. www.newscientist.com/article/dn1639-fetuses-can-hear-ultrasound-examinations-.html. Accessed 11 May 2006.
3 Miller, M.W., et al. 2002. Hyperthermic teratogenicity, thermal dose and diagnostic ultrasound during pregnancy: implications of new standards on tissue heating. International Journal of Hyperthermia. 18(5): 361–84.
4 Edwards, M.J., R.D. Saunders and K. Shiota. 2003. Effects of heat on embryos and foetuses. International Journal of Hyperthermia. 19 (3): 295–324.
5 Milunsky, A., et al. 1992. Maternal heat exposure and neural tube defects. JAMA 268(7): 882–85.
6 Miller, M.W., et al. 2002. Hyperthermic teratogenicity, thermal dose and diagnostic ultrasound during pregnancy: implications of new standards on tissue heating. Int J Hyperthermia 18(5): 361–84.
7 "The ultrasound procedure: Physical effects and research." Birth. www.birth.com.au/class.asp?class=6610&page=5. Accessed 23 Sept 2006.
8 Barnett, S.B. "Can diagnostic ultrasound heat tissue and cause biological effects?" In S.B. Barnett and G. Kossoff, eds. 1998.
9 Barnett, S.B. (2001, July). Intracranial temperature elevation from diagnostic ultrasound. Ultrasound Medical Biology. 27(7); 883-8.
10 Olson, C.D. (2009).