Premature birth can cause long-term health problems
Stress in the six months before conception increases the risk of giving birth prematurely, research suggests.
Mothers who experience a death or serious illness in the family before falling pregnant are more likely to have a preterm baby, a study found.
The link was particularly strong in mothers who had experienced "severe life events" in older children, the Human Reproduction journal reported.
Effects of stress hormones in the womb, may explain the findings, experts said.
Records from 1.35m mothers in Denmark were analysed in the study, which comes on the back of earlier research suggesting stress during pregnancy is a risk factor for premature birth.
Serious illness or death in older children in the six months prior to conception, increased the risk of birth before 37 weeks by 23% and the risk of birth before 33 weeks by 59%.
Death or illness in close relatives, such as parents, siblings or a partner in the last six months before becoming pregnant increased the risk of birth before 37 weeks by 16%.
Mothers who had experience stress and were also missing a parent or who did not have a partner were significantly more likely to have a premature birth, the study showed.
The researchers suggested several possible explanations for the findings including increased levels of stress hormones or lifestyle changes in response to stress, such as increased consumption of tobacco or alcohol.
Study leader, Professor Philip Baker, from the Maternal and Fetal Health Research Group at the University of Manchester said most research to date had focused on stress during pregnancy.
"Yet these results suggest that the impact could be greater in the period preceding conception.
"Although relatively subtle its still a real effect."
He added they had chosen to look at death and serious illness in close relatives as they were undoubtedly very stressful events for anyone.
"This should help our understanding of the causes of premature birth," he added.
Tommy's, the baby charity, who funded the research said around 50,000 babies are born prematurely each year in the UK, putting them at risk of long-term health problems such as chronic lung disease, learning difficulties and blindness.
Ronald Lamont, consultant obstetrician at North West London Hospitals NHS Trust and spokesman for the Royal College of Obstetricians and Gynaecologists, said the results were a "little surprising".
"It's quite interesting and fits with what we knew about stress during pregnancy.
"The fact that the risk of pre-term birth was increased if pregnant women were missing a parent or partner shows the importance of close support."