Children whose biological fathers died while they were in the womb were twice as likely to become obese as adults, because of the stress of bereavement on their mother, the study authors said.
But children also had an increased risk of adult overweight or obesity if their mothers experienced the death of a close relative up to six months prior to their conception.
A mother's response to stress apparently has long-term effects on the child she carries, said study senior author Carsten Obel, an associate professor of public health at Aarhus University in Aarhus, Denmark.
"That maternal stress can influence the development of the fetal stress system seems quite plausible," Obel said. "Cortisol -- the end product of the stress system -- influences the storage of fat, and if this system is programmed to more storage early in life this may very well be a factor in development of obesity."
The researchers based their findings on the medical records of nearly 120,000 Danish men born between 1976 and 1993 and subsequently examined for military service between 2006 and 2011.
To examine the effects of stress, they focused on men born to mothers who lost a close relative just before or during their pregnancy. Close relatives included their partner, another child, a sibling or the mother's parents.
"Our reactions to stress are dependent on personality and other factors, but losing a close relative is believed to be a stressful event to everyone," Obel said.
Young men whose mothers had been bereaved had different degrees of increased risk of overweight and obesity depending on the relationship of the relative to the mother, the researchers found.
Overall, men had a 15 percent higher risk of being overweight if their mothers experienced the death of a close relative in the months prior to conception. They had a 13 percent higher risk of being overweight if a close relative died while they were in the womb.
If the woman had lost her husband, her son had double the risk of becoming overweight or obese in adulthood, the findings showed.
The researchers found no association between adult overweight or obesity and any stress a mother experienced following the birth of her child.
This latest study, published online recently in the journal PLoS ONE, adds to a growing stack of evidence that stress before and during pregnancy can have a long-term impact on the child's health in adulthood, said Dr. Youfa Wang, chair of epidemiology at the School of Public Health and Health Professions at the University at Buffalo in New York.
"If mothers are under psychological or physical stress, that may impact the interchange of nutrients and biochemicals between the mother and the fetus," Wang said.
This interplay can affect the development and future function of the baby's organs. "For example, some organs may become more efficient. Later on, when those babies grow up as adults, those physical changes will put them at some risk of certain diseases," he said.
However, it's difficult to say how intense the stress must be to have an effect on a developing fetus, Obel said.
"We do not know if more moderate levels of stress as well as indicators of physiological stress may have a similar effect, but we are currently testing this out," he said.
Also, experts point out that this research doesn't prove that grief during pregnancy predisposes an unborn child to obesity. Many other factors may come into play.
In the meantime, women who are pregnant or attempting pregnancy should do their best to limit stress in their lives, Wang said.
"They need to be aware that during pregnancy, there are many long-term consequences for their children," he said. "When these children are born, as adults they may need to be more mindful of how to protect themselves and reduce their risk for certain chronic diseases."
For more about obesity, visit the U.S. National Library of Medicine.