What is prenatal maternal stress (PNMS)?

The majority of women know that cigarettes, alcohol and drugs can affect their growing baby during pregnancy. What they may not know is that stress during pregnancy is also a risk factor that may have significant consequences for the developing fetus. In fact, stress during pregnancy may result in lasting effects on the infant’s health status, the development and function of the infant’s immune system; and the cognitive development of the infant.

Chronic stress versus acute stress

Chronic stress, such as unemployment, ongoing familial or marital discord is a type of stress that has true consequences on the pregnant mother and her fetus. However, most human and animal research has focused on acute stress.

Acute stress refers to a sudden stressful change in one’s environment or circumstances. In animal research, acute stress may be obtained by emitting a sudden loud noise or by holding the animal in a contained environment for a specific time. In human research, such tests are of course impossible to do. But some life events, such as the death of a spouse, war or a natural disaster provides conditions very similar to a lab’s.

Objective versus subjective stress

The effects of stress are not only a function of exposure to a stressful circumstance but also of the individual’s perception of the stress. In order to fully appreciate the effects of prenatal stress, it is important to understand the difference between objective and subjective stress.

Objective stress

Objective stress refers to the measurable amount of hardship experienced by an individual.
Factors that may contribute to this stress include:

  • Number of days without electricity following a storm
  • Incurred losses (such as money or property)
  • Change to daily routines
  • Possible threats (security or safety issues)
  • Length of the event

Subjective stress

Subjective stress quantifies an individual’s reaction to a traumatic event. An individual can have a high objective stress, but a low subjective stress. For example, feeling you’re in control of a situation or keeping your cool in the middle of a storm. Subjective stress can be evaluated by questionnaires, which determine the psychological response to the events.

Surprising results from the Suzanne King's research team on PNMS, indicate that objective stress has more of an affect on fetal development than subjective stress.

How is stress communicated from mom to fetus?

Understanding how prenatal maternal stress can affect a developing fetus requires some knowledge about the biology behind the stress response. Response to stress involves a number of organs and systems within the body; from the brain to specialized organs, such the adrenal glands, which are adjacent to the kidneys.

The process begins with a stressor stimulating the brain, which evaluates the threat and processes it into an appropriate response, physiological and behavioural. This results in the secretion of corticoids, such as cortisol, and glucocorticoids from the adrenal glands into the bloodstream. The corticoids are molecules, which trigger the “flight or fight” response of an individual to stress

Cortisol is the link between prenatal stress and infant outcomes. Prenatal maternal stress is associated with increased levels of cortisol in the mother. It is believed that this molecule has a direct effect on the fetus. Moreover, because a linear relationship exists between maternal and fetal cortisol levels, relatively small increases in maternal cortisol are equivalent to relatively large increases in fetal cortisol.

When is a fetus most susceptible to prenatal maternal stress?

Timing is everything, especially for the effects of prenatal maternal stress (PNMS). New research findings show that the first two trimesters are the most sensitive to prenatal stress. Two periods are especially crucial :

  • At week 10, the embryo becomes a fetus and it begins to move. The vital organs now have a solid foundation. During this time, the brain will produce almost 250,000 new neurons every minute. This is called neurogenesis
  • During weeks 24 and 30, nerve cell connections are occurring. Guided by chemical signals, nerve processes seek out their target and establish contact. Communication between neurons begins. At birth, there are an excess of nerve connections, those that are not used will degenerate. This is called synaptogenesis

Exposure to extreme stress during these critical periods of pregnancy will influence which developing structures are affected and therefore determine the physical, cognitive or behavioral outcome.

For example, research conducted by Suzanne King and her team following the Quebec 1998 ice storm, shows:

  • That high levels of objective PNMS were associated with lower intellectual and language abilities of two-year old children. The effect of objective PNMS was observed ONLY in the first or second trimester-exposed toddlers.

Effects of stress on the fetus

The exposure of an expectant mother to stressful situations can influence the cognitive, behavioural and physical development of the children. In addition it may result in an increased risk of other mental health disorders, such as autism and depression.

Effects on cognition

Many recent human research findings have shown that acute prenatal stress affects children’s cognition – or ability to think. These studies involve evaluating the mothers’ stress levels during pregnancy and following their children as they grow. To determine the children’s cognitive abilities, tests are administered which determine their intellectual and language skills.

Research examples:

  • A study conducted by the Douglas Institute monitored pregnant women and their children during and following a severe ice storm. The children exposed to high levels of objective prenatal stress, exhibited poorer cognitive and linguistic abilities relative to the children exposed to low levels. This trend was observed at 2, 5 ½ and 8 ½ years of age.
  • A study conducted in The Netherlands correlated prenatal stress with infant performance at three and eight months. Their findings showed that mothers who had high levels of the stress hormone, cortisol, during mid-pregnancy, had children who scored lower on cognitive tests relative to their peers. This effect was more obvious for the older children than the younger.
  • A study conducted in Rochester, NY, evaluated mothers who were admitted to a clinic serving high-risk patients. This study showed that stress or anxiety in pregnancy is associated with increased cortisol in the mother. In addition the researchers suggested that this prenatal exposure to cortisol predicts a lower cognitive ability at 17 months.

Effects on behaviour

There is consistent evidence demonstrating that prenatal maternal stress affects the behaviour of children. Children exposed to this stress may show difficulties with paying attention, and have aggressive attitudes.

Research examples:

  • The Ice Storm Project conducted by Suzanne King’s team at the Douglas Institute monitored pregnant women and their children during and following a severe ice storm. Their analysis showed that children exposed to a high level of objective stress (lien vers def.), subjective stress or both had behavioural difficulties: they had higher attention problems during kindergarten. Also the children of mothers with high level of subjective stress (lien) were more likely to have depressive problems.
  • A study from Tulane University, evaluating the post-hurricane Katrina effects showed that 55. 3 percent of children living in subsidized housing in Mississippi experienced emotional or behavioural difficulties not present before the storm. Twenty-one months after Katrina, 25.4 percent of Mississippi children and 37.1 percent of children in Louisiana had behavioral disorders.
  • A study conducted in Rochester, NY, evaluated mothers who were admitted to a clinic serving high-risk patients. This study suggests that prenatal stress may be associated with increased fear reactivity in boys.

Physical effects

Recent research findings have demonstrated that obstetrical complications, low birth weight, and delayed physical development may all be influenced by prenatal maternal stress.

Research example:

The Ice storm project conducted by the Douglas Institute monitored pregnant women and their children during and following a severe ice storm. Their findings showed that:

  • Women who were exposed to the Ice Storm before conception or in their first trimester of pregnancy had significantly more obstetrical complications.
  • Researchers involved in the Quebec Ice Storm project were able to correlate the time of stress with certain physical changes, namely the development of fingertip ridges. Because fingerprint development occurs during the second trimester of gestation when critical brain structures are also developing, any abnormalities observed with the fingerprints, may have important implications for psychological development as well.
  • The researchers’ findings showed that asymmetries were observed in the fingerprints of children whose mothers were without electricity during 14 and 22 weeks of pregnancy. Although the objective severity of the stress exposure was moderately correlated with the asymmetry, when the all the data was analyzed, the women’s degree of subjective stress emerged as a higher predictor of asymmetry.

These results highlight the sensitivity of the developing fetus to the environmental conditions and emotional status of the mother.

Effects on mental health

Prenatal exposure to stressful events is associated with an increased risk of autism, schizophrenia and depression of the children.

Research examples:

  • In a study from Ohio, mothers who reported stressful life events, such as job loss or spousal death, were more likely to have children with autism.
  • In a study from Finland, individuals who were in utero when their fathers died were significantly more likely to be treated for psychiatric disorders.
  • The incidence of schizophrenia was increased in children who were exposed in utero to severe tornados.

Possible interventions for prenatal maternal stress

The exposure of an expectant mother to acute stress can be reduced by creating relaxing environments. However, this is not always easy to accomplish, especially in the face of natural disasters. Researchers are actively studying how to reduce the impact of these occurrences, not only for expectant mothers and children, but also for society as a whole.

Possible interventions before birth

Interventions may help moderate the response of an expectant mother during a time of crisis. The goal of these interventions is to remove the anxiety and provide a sense of control.

Essential elements for successful interventions include providing:

  • A sense of safety
  • A calming influence
  • A sense of self and community efficacy
  • A sense of connectedness
  • Hope

Crisis interventions may include therapy, problem solving advice, self-help groups, education and psychotherapy. Expectant mothers may be particularly concerned about the well being of their baby, supplementary ultrasounds, testing and education will help to ease their worries.

The following public health measures have been shown to be helpful during crisis:

  • Promoting mental health through prevention
  • Providing a continuum of services, from prevention to intervention
  • Creating community collaboration and school-linked services
  • Creating culturally sensitive services

Possible interventions after birth

Although few human research studies have been conducted in this area, researchers and clinicians believe that some effects of prenatal stress are reversible. Given the right postnatal environment and support, children may overcome some of their initial difficulties. For example, one study showed that secure child-parent attachment eliminates the adverse effects of prenatal cortisol exposure on cognitive development.

Research projects at the Douglas

Currently, a key research project at the Douglas is directly evaluating the effects of prenatal maternal stress.

Project ice storm


Project Ice Storm, directed by Suzanne King, Ph.D. studies the effects of exposure to events surrounding a natural disaster, which offered the opportunity to assess the severity of different forms of disaster related stress. The January 1998 ice storm in Quebec resulted in power outages ranging from a few hours to more than six weeks. The Insurance Bureau of Canada listed this ice storm as the most costly natural disaster in Canadian history.

Researchers were able to obtain both subjective and objective measures of prenatal maternal stress that were independent of other maternal characteristics for a large number of pregnant women. This study is unusual and unprecedented because the precise period of pregnancy during which the women were exposed to the stressor can be accurately pinpointed. Furthermore, the sample of women recruited is homogeneous as they all come from the same socioeconomic and geographic region.

The project's main aim is to investigate the impact of prenatal maternal stress on peri-natal and pre-natal development and later childhood development.

Recent findings

King and her team have shown significant differences in the level of intellectual and linguistic development as a function and severity of the mothers’ objective exposure to the ice storm; toddlers whose mothers were exposed to high levels of objective prenatal maternal stress had lower IQ scores and understood and spoke fewer words compared to toddlers whose mothers were exposed to low levels of objective stress.

More recently King and colleagues concluded that pregnant women’s exposure to a moderately stressful event during the period of fingertip development results in greater asymmetry in their children, especially when combined with higher levels of maternal subjective stress.

Their results highlight the sensitivity of the fetus to the environmental conditions and emotional status of the mother.

These findings have implications for public health and safety personnel managing natural and man-made disasters, and those who work to protect pregnant women from situations of acute stress.

Project Ice Storm is ongoing and new findings are continually forthcoming.

Christine Zeindler