Theories vary on exactly why stress during pregnancy would affect the baby, but some center around a hormone called cortisol. Cortisol tends to be elevated in people feeling stressed. Some elevation is normal during pregnancy but above average elevations could be related to miscarriage. Some scientists believe that this elevated cortisol could cross the placenta and interfere with development.
In a 2008 study, researchers administered the 12-item General Health Questionnaire (GHQ) about stress during pregnancy also found that women reporting higher levels of stress seemed to have an 80% higher risk of stillbirth when compared to women with intermediate levels of stress. Adjustment with a variety of other factors, such as mother's age or health risk factors, did not change the results.
In addition, a 2006 study found evidence that cortisol levels increased above average for pregnancy meant an increased risk of early miscarriage, such as within the first three weeks after conception. A 2002 study also linked depression as being a risk factor for further miscarriages in women who had recurrent miscarriages.
Looking at preterm birth, which is a risk factor for newborn infant loss, a 2003 study examined 1,962 women and found that those who reported high counts of anxiety were more likely to experience preterm labor and subsequent birth. Other studies had previously had similar findings showing stress as a risk factor for preterm birth and low birth weight, with outcomes varying by the level of stress and timing of the stressful events. A 2003 review found that stress during early pregnancy was most likely to be associated with "shortened gestation."
Evidence Against a Link
Not every study looking at stress during pregnancy has found evidence of a link with miscarriage. A 1998 study found no increased risk in women who had elevated cortisol and other hormonal markers associated with stress.
Another 2003 study found that women reporting high stress in early pregnancy did not have an higher risk of miscarriage when looking at stress alone, but the study did find that women under stress were more likely to use drugs like cigarettes and marijuana, which might be risk factors for miscarriage independently.
With these studies in mind, one could argue that the exact link between pregnancy stress and miscarriage is not fully understood or accepted.
Where It Stands
Currently, no one is able to say conclusively that "stress causes miscarriages," but it also doesn't seem accurate to say that it's a myth that stress can cause pregnancy loss. The truth is that it is possible that anxiety and stress could be linked with miscarriage but the evidence is too unclear to draw conclusions.
It is unlikely that normal everyday stress and worries, such as worrying about your finances or deadlines at work, would have any effect on pregnancy, but it is possible that major levels of stress could cause miscarriage or later pregnancy loss.
But regardless of the link with miscarriage, stress during pregnancy may affect the baby in other ways also and it's always a good idea to make stress management a priority in your life. Stress may be unavoidable for many people, especially if you're dealing with something like infertility or recurrent miscarriages, but it might be a good idea to look into doing whatever you can to alleviate your anxiety and to get your mind off things. In doing so, you might improve your odds for a healthy pregnancy as well as your overall health. Simply put, there is no downside to incorporating more relaxation and to addressing any anxiety disorders that might be affecting your quality of life.