Stress induces a fight or flight effect in the body. When stress is at its highest, physical reactions can include reduced blood flow to extremities, decreased metabolism, and increased focus. During pregnancy, prolonged or extreme stress will result in immediate effects on the body, but the long-term effects may be life changing. Researchers have linked stress early in pregnancy to increased risk of miscarriage.

According to one 1997 study published by Dr. Marc B. Schenker of UC-Davis medical school in Sacramento, female attorneys who logged 45 hours or more during the first trimester of pregnancy increased their risk of miscarriage three-fold. While job stress may be the most common stressor for pregnant women, other stressors like drinking alcohol, using drugs, or smoking can also increase stress and thus risk of miscarriage. The same study noted women who drank seven or more alcoholic drinks per week increased miscarriage risk by 7 times.

In another study completed by Dr. Pablo Nepomnaschy of Triangle Park, North Carolina, women with higher blood Cortisol levels were 2.7 times more likely to suffer miscarriage. The study included 61 women in which 22 pregnancies were reported with 10 elevated Cortisol levels. Of the pregnant women with elevated Cortisol levels, nine miscarried, 90%. Four out of twelve pregnancies in women with no increase in Cortisol levels were miscarried, 33%.

For some women, health before pregnancy may greatly impact the effect of stress on fetal implantation and growth. The Miscarriage Association sponsored a study in the UK to study the effect of lower than normal body mass index (BMI) on risk of miscarriage. During the study, it was found that women who were underweight were more likely to miscarry, but researchers also noted the effect of stress on fetal health was also a huge factor. According to the study, increasing weight or BMI to a normal level before becoming pregnant could impact the effect of stress on miscarriage risk.

Centuries ago, stress was defined far differently than it is today. Stress was often associated with reduced food supplies, war, famine, and disease. If stress levels rose in a population of people, pregnancies may have been naturally miscarried to protect women and infants. Rewriting genetic reactions takes far more than a few centuries. Today, stress is not commonly associated with these ancient factors, but the body may react in the same way thus the rise in Cortisol levels and resulting miscarriage.