Girls were more likely after stress
The researchers, from John F Kennedy Institute and the Danish Epidemiology Science Centre and colleagues from the Centre for Research in Health and Social Statistics and the Danish Epidemiology Science Centre, analysed the male-to-female ratio of 3,072 babies who had been conceived amid traumatic events.
They found that the proportion of boys was 49% among those who had been traumatised as opposed to 51.2% among those who had not.
The reason for this could be due to changes in sexual behaviour following traumatic events, changes in hormone levels, reduced semen quality or increased rates of miscarriage of male babies.
Disaster impact
The researchers, led by Dr Dorthe Hansen, had seen earlier studies that suggested exposure to smog, floods or earthquake led to a reduction in the proportion of males born.
They decided to examine whether or not exposure to "severe life events" before and during the period of conception could affect the sex of the baby.
First they gathered data on all Danish women who gave birth between 1980 and 1992, and then cross-referenced them with severe life events.
These were defined as the woman's partner or elder children dying, getting cancer or suffering a heart attack.
In total, there were 3,072 single births among those exposed to a severe life event and 20,337 among those who were not.
Crucial timing
Those who experienced the event closest to the time of conception were least likely to have a boy.
Publishing their findings in the British Medical Journal, the researchers said they were fairly reliable.
"The study cohort was large, and the registration of death, cancer and the offspring's sex is known to be accurate and complete," they said.
"The results are consistent with results for natural disasters."