Stress carries with it many health problems, including increased risk of heart disease and depression. For pregnant women, however, this list is longer and includes risks for the child - including premature birth, low birth weight and developmental problems. Now, a new study links maternal stress to an increased risk of asthma for offspring.

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The new study suggests that maternal stress in pregnancy can increase risks of asthma and allergies for the baby.

The study, which was conducted using pregnant mice, is published in the American Journal of Physiology.

It is already well known that keeping stress levels low in pregnancy is important for both mother and baby. Medical News Today recently reported on a study suggesting that yoga during pregnancy can keep maternal stress levels low, preventing anxiety that can lead to postnatal depression.

And even before pregnancy, stress has been linked to an increased risk of infertility.

In this latest study, the researchers, from the Harvard School of Public Health in Boston, MA, found that stress in pregnant mice was linked to an increased risk of allergy-induced asthma in their pups.

According to the Centers for Disease Control and Prevention (CDC), asthma is one of the most common, long-term childhood diseases. In 2010, 1 in 12 adults and 1 in 11 children had asthma. Additionally, in 2009, 3,388 people died from asthma.

Because this is such a widespread, costly condition that has no cure, the prevention of asthma - if possible - is extremely important.

Mother's stress hormones can cross placenta

The researchers note that glucocorticoids (GCs) are stress hormones that naturally occur in the body and help to keep inflammation down. As such, synthetic versions - such as prednisone, dexamethasone and hydrocortisone - are frequently used in the wake of allergic reactions.

Fast facts about asthma in the US
  • Each year, asthma costs the US $56 billion
  • In 2009, the average yearly cost of care for a child with asthma was $1,039
  • In 2008, asthma caused 10.5 million missed days of school.

However, when released in the body as a stress response, these same GCs can also lead to inflammation and increase allergic responses to environmental irritants, rather than help fight them off.

In pregnant women, GCs are naturally elevated, increasing risk for an adverse allergic response if stress further increases these levels. To further investigate, the team looked at whether the increase in GCs due to maternal stress in pregnant mice could lead to asthma development in the offspring.

One group of pregnant mice was exposed to a single instance of stress while a second group was given dexamethasone to reproduce the effects of stress. Meanwhile, a third group was given a steroid-inhibitor - called metyrapone - that blocks the release of stress hormones.

A fourth group acted as a control group and was not given any interventions.

The researchers found that high concentrations of stress hormones - corticosterone (CORT) - in the mother were able to cross the placenta and increase CORT levels in the fetuses, which could lead to higher risks of developing asthma and allergies.

After birth, the researchers exposed all of the mice to allergens. Commenting on their findings, the researchers say:

"Only the offspring of stressed mothers demonstrated increased asthma susceptibility compared with non-stressed mothers. We also demonstrated that a single episode of stress significantly elevated maternal stress hormone levels."

They further conclude that their results "indicate that maternal stress can play a role in the initiation of asthma by increasing asthma susceptibility in offspring."

Study limitations

The study also carries with it certain limitations. Firstly, the team used a stress hormone analog called dexamethasone rather than CORT. Although these two compounds are very similar, the researchers say they are not identical.

Specifically, dexamethasone is more potent and crosses the placenta without degrading, compared with CORT. The researchers say that because of these differences, the use of dexamethasone as an injection may not exactly recreate the effects of rises in CORT following stress.

Additionally, the team notes that their model cannot differentiate between prenatal and postnatal effects of maternal stress, which could have different implications. For example, stress could alter maternal behavior or breast milk, which could prompt changes in the neonatal immune system.

Still, the researchers note that because "inflammation typically includes a stress hormone response, the results also suggest a common pathway by which various injurious exposures during pregnancy might increase offspring susceptibility to asthma."

Medical News Today recently reported on a study that suggested smoking bans are linked with a decline in preterm births and asthma.