Children whose mothers were stressed out during pregnancy are vulnerable to mental and behavioural problems like ADHD, mounting evidence suggests.
Infants were anxious and fearful
Latest UK research by Professor Vivette Glover of Imperial College London found stress caused by rows with or violence by a partner was particularly damaging.
Experts blame high levels of the stress hormone cortisol crossing the placenta.
Professor Glover found high cortisol in the amniotic fluid bathing the baby in the womb tallied with the damage.
The babies exposed to the highest levels of cortisol during their development had lower IQs at 18 months.
The same infants were also more likely to be anxious and fearful, she told a conference of the Royal College of Psychiatrists.
Professor Glover said: "We looked at what stresses were most harmful.
"We found that if the woman had a partner who was being emotionally cruel to them while they were pregnant it had a really significant effect on their baby's future development.
"It really shows that the partner has a big role to play."
The work suggests maternal stress is a true risk factor in its own right, although Professor Glover acknowledged that genetic factors and home environment after birth would also have an impact on a child's development.
She said most babies grow up unaffected by a stressful womb environment.
However, she said maternal stress increases the risk of a range of problems - it doubles the risk of attention deficit hyperactivity disorder (ADHD), for example.
"We should be screening women in pregnancy for stress and intervening.
"It has big public health implications. About a million children in the UK have neurodevelopmental problems - ADHD, cognitive delay, anxiety and so on.
"About 15% of this might be due to antenatal stress.
"If we could reduce the mother's stress while she is pregnant we might be able to potentially improve the outcome for about 150,000 children," Professor Glover said.
Dr David Coghill, senior lecturer and honorary consultant in child and adolescent psychiatry at the University of Dundee, said pregnant women should not be "unduly concerned" by the findings.
He explained: "We are talking about here is extremely high levels of stress and distress.
"Stress is a normal factor of daily life and is something that the body copes with very well.
"However, it is a warning for people who may be facing more severe stresses and for those around women who are pregnant that increasing stress levels above what is normal for a person is not a good thing to do at that time."
Professor Glover has submitted her work for publication in a peer-reviewed journal.