Moderate stress during pregnancy does not harm the unborn child but can instead aid its later advancement, US research suggests.
Earlier research suggests stress can harm an unborn baby
The team asked 137 healthy women with low-risk, normal pregnancies to report on their stress between the 24th and 32nd week of pregnancy.
The study in Child Development found the children of those who reported more stress were more advanced at age two.
Earlier studies suggest stressed out mothers can pass it on to their babies.
They also suggest high stress levels can lead to restricted growth and birth defects in the unborn child.
And the researchers at Johns Hopkins Bloomberg School of Public Health in Baltimore expected to find that distress during pregnancy would be linked to bad behaviour and temperamental dysfunction in children at age two.
Research author development psychologist Professor Janet DiPietro said: "We thought maybe they would show some signs of being difficult or of emotional dysfunction. Instead we found the reverse was true."
There were two possible explanations for this, she said.
Women who have high stress levels would be generating more of the stress hormone cortisol.
It is one of the chemicals produced naturally in the body when stress triggers a 'fight or flight' response.
"Cortisol has a bad rap as the stress hormone - but every organ in the body needs cortisol to develop properly.
"It could be enhancing the development of organs before birth," said Professor DiPietro.
Alternatively, said Professor DiPietro, it could be that the type of women they are affects the way they bring up their child.
She said: "These are normal, mostly working women who are juggling the demands of every day life. These are women that challenge themselves - they are not severely depressed or in receipt of psychiatric care.
"Maybe these women have higher stress levels because they challenge themselves, and because they challenge themselves they challenge their children after birth too prompting faster development."
In addition the team also found prenatal stress did not affect the children's ability to handle their own emotions or control their behaviour.
However the children of women who had reported negative feelings about being pregnant appeared to show poorer behavioural and emotional regulation.
Again it was not clear from the research whether this was caused by biological factors or the way the child was treated after birth.
Professor DiPietro said although the team found the results unexpected they were pleased with them.
She said: "I'd like all pregnant women to heave a big sigh of relief - they hear all these awful stuff about everything they do.
"Now they can stop worrying about worrying.
"The reason to avoid stress is not because it's hurting the baby but because it is hurting you."
Spokesman for the Royal College of Obstetricians and Gynaecologists Patrick O'Brien said the findings of this well-designed study were very interesting although they conflicted with earlier research.
"It will be very reassuring to the many women who do experience a reasonable amount of stress during their pregnancy for whatever reason.
"There are so many people that come to me and say they are worried about how working long hours or a problem with a boss is affecting their baby.
"The message is that a moderate amount of stress will not harm the baby at all and in fact might do it some good," he added.