April 6, 2001 -- Pregnancy is one of the worst possible times to be under excess stress. Unfortunately, some women can't avoid it.
In 1994, for example, people living in Northridge, Calif., and the surrounding area experienced an earthquake measuring 6.7 on the Richter scale, a quake considered one of the costliest natural disasters in U.S. history. Experts say some women who were in the early stages of pregnancy at the time of the quake may have delivered early due to the stress of the event.
One theory is that the early stress activates a "clock" in the placenta that controls how long a pregnancy will last, says Laura M. Glynn, PhD, a researcher at the University of California, Irvine.
"The signals come from the fetus, but are transmitted back and forth across the placenta," she says.
Glynn and colleagues studied 40 women who were pregnant or had delivered a baby within six weeks after the Northridge quake, which caused 57 deaths and more than 1,500 injuries. Women were asked to rate the earthquake on a four-point scale ranging from "not upsetting at all" to "extremely upsetting."
Women who were in their first trimester of pregnancy when the earthquake happened were more likely than those in later stages of pregnancy to rate the experience as "extremely upsetting." Women in the third trimester were most likely to rate it as "not upsetting at all."
Glynn says the findings may indicate a decreased response to stressful events that occurs as a pregnancy advances. This could be a natural form of protection that ensures the birth will occur regardless of what else is happening around the mother.
"Her emotional responses to the stress are less later in pregnancy, but it's also the case that her [bodily] responses are less," Glynn says. "So if you stress a woman late in pregnancy, her blood pressure responses are not going to be as big. If you give an infusion of a substance that should create a [stress] response, you don't see that response."
Other studies have shown that when traumatic or emotional situations occur, levels of the body's stress hormones, including cortisol, increase.
"I think our study fits with that in the sense that as far as [birth defects] go, the fetus itself is vulnerable early on, separate from the mother," Glynn says. "Now you can layer on to that the stress transmitted by the mother to the fetus. So there's kind of a double vulnerability going on: one that is related to the rate of development of the fetus, and the other one related to the mother's responses to the stress."
In Glynn's study, women who were exposed to the earthquake in the first trimester gave birth at an average of 38.05 weeks, while those who were in their third trimester delivered at an average of 38.99 weeks. A typical pregnancy is 40 weeks.
While the differences are not drastic, Glynn says they do show the potential for premature delivery. With more research, she hopes the information may be useful to researchers trying to develop interventions against premature birth. One of those interventions may be for women to avoid stressful events during the first trimester.
But experts say studies have not consistently showed that stress, even severe stress, always leads to bad outcomes for pregnant women.
Mark Klebanoff, MD, says studies of women who were pregnant during a war in Israel -- which should have been a stressful time -- showed no increase in prematurity rates. Klebanoff, director of the National Institute for Child Health and Development's Division of Epidemiology, Statistics, and Prevention Research in Bethesda, Md., says while the studies are all interesting, individual people will have individual responses to stress that can't always be generalized or predicted.