Stress and anxiety during pregnancy could cause women to have smaller babies, researchers have discovered.
They believe the effect is caused by reduced blood flow through the arteries that feed the uterus. Doctors from the Queen Charlotte's and Chelsea Hospital in London studied 200 expectant mothers.
They found pregnant women who were more anxious or stressed had "significantly abnormal patterns of blood flow through the uterine arteries". The findings, published in the British Medical Journal, suggest that "the psychological state of the mother may affect foetal development and therefore birth weight".
The women were asked to rate their stress levels using a questionnaire. The researchers carried out ultrasound scans to test the blood flow through arteries. They found that those who were the most anxious during pregnancy had significantly abnormal blood flow patterns.
A "resistance index" measurement was taken of the extent to which the blood flow was impaired.The researchers found that of the most anxious group, 28% had a resistance index high enough to be of "clinical concern". Only 5% in the less anxious group had similarly impaired uterine artery blood flow.
One of the study authors, Dr Vivette Glover, a reader in perinatal psychobiology, said the size differences recorded were small - around 12%. But she said research had indicated that even small variations could have a potentially serious impact on health in later life.
Small babies are more likely to develop coronary heart disease, diabetes and depression in later life. Dr Glover said more research was needed to determine why stress should impact on development.
"One possible reason is that stress raises the levels of hormones such as nor-adrenalin, which is known to constrict the blood vessels and decrease blood flow," she said.
"Another possible mechanism is that anxiety may have a chemical effect on the development of the blood vessels earlier on in pregnancy."
She added that it was not known when stress would have a worse effect on a foetus' development. The study looked at foetuses in the late stages of development, but it could have more of an impact in the early stages when the placenta is being formed, said Dr Glover.