Keeping Cool Through Pregnancy

By Elaine Stillerman, LMT
May 29, 2009

Keeping Cool Through Pregnancy

By Elaine Stillerman, LMT
May 29, 2009

There is a sign posted outside of my gym's sauna that warns pregnant women about using the heated room. This same sign should be posted outside of steam rooms and hot tubs at health clubs, gyms, spas and resorts across the U.S. But what a lot of pregnant women don't understand is why this warning is provided in the first place.

According to a 2003 study reported in the International Journal of Hyperthermia, "Hyperthermia during pregnancy can cause embryonic death, abortion, growth retardation and developmental defects."1 The study also asserts that an increase in maternal temperature of even 2 C (3.6 F) for a 24-hour period also can cause a "range of developmental delays."2

An equally revealing study reported in JAMA states 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."3 Hot tubs and heated baths pose more grave dangers than other heat sources because immersion disrupts the body's attempt to cool through perspiration. The fetus cannot escape the increased temperatures while still in utero. But there also is another serious problem that pregnant women have to be made aware of - the heat of ultrasound.

There are a lot of important reasons to maintain a normal body temperature during pregnancy. (The average temperature is 98.6 F.) Proper body temperature is essential to proper enzyme reactions. Temperature can affect the actual shape of the proteins that manufacture enzymes, and when their shape is thwarted due to increased temperature, enzyme reactions become less and less efficient until they are permanently damaged.4

We have different ways to stay cool and keep our core temperature stable. We shiver when we are cold and perspire when we are warm. But the fetus cannot do that. However, fetuses do have a defense against rising temperatures. Each fetal cell contains heat shock (HS) proteins that temporarily stop the formation of essential enzymes when temperatures become too high.5 Unfortunately, HS fails to protect the fetus in later pregnancy, and once normal protein synthesis is delayed or suspended, normal development may not occur.6

Ultrasound heats the bones of the developing fetus at a different rate than other tissues. The older the fetus and the more calcification within the skeletal system, the more heat these young bones absorb and retain. During the third trimester, the fetal cranium can heat up 50 times faster than its surrounding tissue.7 This can subject the parts of the brain near the skull to secondary heat that can continue after the test is completed.

We have to conclude that when a pregnant woman is exposed to heat, whether it's from a water source, maternal temperature or ultrasound, the fetus can suffer devastating consequences. In order to prevent this from occurring, pregnant women need to be made aware of the potential dangers and risks of all of these heat sources.

It's one thing to be a "hot momma." It's quite another thing to endanger the baby when this can easily be prevented.


  1. Edwards MJ, Saunders RD, Shiota K. Effects of heat on embryos and fetuses. Int J Hyperthermia, 2003;19(3):295-324.
  2. Ibid.
  3. Milunksy A, et al. Maternal heat exposure and neural tube defects. JAMA, 1992;268(7):882-5.
  4. Rodgers C. Questions about prenatal ultrasound and the alarming increase in autism. Midwifery Today, Winter 2006;(80):16-19,66-67.
  5. Ibid.
  6. Edwards MJ. Apoptosis, the heat shock response, hyperthermia, birth defects, disease and cancer. Where are the common links? Cell Stress Chaperones, 1998;3(4):213-20.
  7. Barnett SB. "Can diagnostic ultrasound heat tissue and cause biological effects?" In S.B. Barrett and G. Kossoff, eds. Safety of Diagnostic Ultrasound. Canforth, UK: Parthenon Publishing, 1998.