A baby’s weight at birth is linked to the mother’s pattern of stress hormone before she even becomes pregnant, say researchers writing in the journal Health Psychology.

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The study is the first to suggest women’s pre-pregnancy stress levels may affect the birth weight of their babies.

When we experience stressful and traumatic events, our bodies release a hormone called cortisol. Typically, cortisol levels are highest when we get out of bed in the morning and reduce as the day progresses.

But some people experience much smaller levels of decline during the day – a pattern that has been linked to chronic stress and having a history of trauma, say the study authors, who describe the pattern as a “flatter diurnal cortisol slope.”

This abnormal pattern of cortisol has also been linked to the progression of cancer, hardening of the arteries, and other diseases, explain the researchers, whose latest work finds it may also predict the risk of having a baby with lower birth weight.

For the study, Christine Guardino, a postdoctoral scholar in psychology at the University of California-Los Angeles (UCLA), and colleagues analyzed data from 142 female participants of a Community Child Health Network study that was investigating how chronic stress affects new parents and their babies.

They selected these particular participants because they had provided daily saliva samples – from which cortisol can be measured – and had also conceived again while taking part in the larger study.

When the researchers analyzed the relationship between the women’s inter-pregnancy cortisol patterns and birth outcomes, they found those with a flatter diurnal cortisol slope tended to have lower birth weight babies, as Dr. Guardino explains:

“We found that the same cortisol pattern that has been linked with chronic stress is associated with delivering a baby that weighs less at birth.”

Every year in the US, over 300,000 babies are born with low birth weight, which is weighing less than 2.5 kg (under 5.5 lbs), putting them at higher risk of dying in infancy, developmental problems and life-long health conditions, including cardiovascular and metabolic disorders.

Maternal cortisol plays an important role in the development of the unborn child, and levels can increase two to four times during a normal pregnancy. But should levels become abnormally high, they can reduce blood flow to the fetus and deprive it of much-needed oxygen and nutrients.

Abnormal levels of cortisol in pregnancy can also affect a child’s response to stress later in life, says Chris Dunkel Schetter, a UCLA professor of psychology, who also worked on the study.

While the link between cortisol during pregnancy and fetal development was known about before, the authors believe their study is the first evidence that a woman’s cortisol pattern even before conception may affect the weight of her baby.

Prof. Dunkel Schetter says the study adds to evidence about the importance of pre-conception health, and she urges women considering having a baby to plan well in advance and ensure they are healthy and aware of the effects that everyday stress can have. She concludes:

Women should treat depression, evaluate and treat stress, be sure they are in a healthy relationship, be physically active, stop smoking and gather family support. All of the things that create an optimal pregnancy and healthy life for the mother should be done before getting pregnant.”

The researchers found that the mothers in the study were affected by stress arising from a range of sources, from financial difficulties and relationships with family and neighbors, to major life events, including death of a family member and incidents of violence and racism.

In a recent article, Medical News Today explains how friendships can improve health and well-being, and how simply having a good friend around during a negative experience may reduce stress.