Now researchers suggest that the two conditions may indeed be linked.
The authors say this is the first study to link a biomarker for stress with delayed conception in normal, healthy women, and they suggest that finding ways to reduce or manage stress may be a low-tech solution for some infertile couples.
"Even when couples just start trying to conceive, people are really stressed out," said the study's lead author, Germaine Buck Louis of the Eunice Kennedy Shriver National Institute of Child Health and Human Development.
"There's an expectation—they want to have their family now," Buck Louis said. "Stress is the one most consistent factor that shows an effect on how long it take to get pregnant, of all the lifestyle factors studied to date."
Repeated failures to conceive month after month could potentially set off a vicious cycle in which it becomes ever more difficult to get pregnant, she said.
The study tracked 274 British women aged 18 to 40 who had just started trying to conceive, following them for six months or until they became pregnant.
The volunteers were given at-home fertility test kits to track their monthly cycles, and on the sixth day they collected saliva samples that were tested for the stress hormone cortisol and for alpha-amylase, which is secreted when the nervous system produces catecholamines, which initiate another stress response.
Although there was no correlation between cortisol levels and the time it took to get pregnant, women with the highest concentrations of alpha-amylase were 12 percent less likely to get pregnant each month than those with the lowest levels.
"This is one more piece of the puzzle that's adding up to the same conclusion: that stress is not necessarily a good thing for our reproductive system," said Alice Domar, executive director of the Domar Center for Mind/Body Health at the fertility center Boston IVF, who was not involved in the research.
The surprising finding was that even low levels of stress can affect conception, said Dr. Sarah Berga, head of obstetrics and gynecology at Emory University, who has studied the effect of stress management on women who are not ovulating.
"This shows that even low levels of stress play a role," she said. "You don't have to get stressed out to the point of losing your period, apparently, to have an impact on your fertility."
Curiously, the odds of conceiving were higher for women who had elevated concentrations of cortisol during the fertile period. Cortisol is secreted when the endocrine system responds to stress; alpha-amylase is part of the "fight or flight" response, usually activated by the sympathetic nervous system in response to acute stresses.
Women who have been treated for infertility say the process can be highly stressful—in fact, all-consuming. Amy Cafazzo, a 38-year-old from Framingham, Mass., said stress-reduction classes at Boston IVF taught her coping skills she still uses—now to cope with her active 19-month-old twin boys.
"When you're going through this, people often say, `Relax, it'll happen,' and you just want to smack them," Cafazzo said. "You can't just relax."
The classes taught breathing techniques, yoga and other relaxation strategies and provided her with a support network of women going through the same experience. Did it actually help her conceive?
"I just don't know," she said. "But I'm a Type A personality that needs an actionable plan, and this gives you something to do, so you feel productive and aren't caught in a downward spiral of stress."