Babies fathered by teenagers are more likely to be unhealthy at birth, a study suggests.
Social factors may be key
The age of the mother has been linked to birth problems, but the Canadian researchers do not know why a younger dad might have an impact.
Specialists from the UK said that teenage fathers were generally less affluent, and might offer less emotional support to their partner.
The study, of 2.6 million mothers, features in Human Reproduction.
All the women were aged between 20 and 29 when they gave birth, and the fathers divided into age groups, so that the babies of teenage dads could be compared with those fathered by older men.
Men aged 20 and upwards did not show any abnormal risk of fathering a child with birth problems.
However, the teenagers had a 13% increased chance of a low birthweight baby, a 17% chance of a small baby and a 15% increased chance of having a baby born prematurely.
The chances of a baby dying within the first year also increased by 41%.
Professor Shi Wu Wen, from the University of Ottawa who led the study, said the findings "warranted further investigation".
"The magnitude of the risks to society could be huge, given the number of births worldwide, if the increases we found are truly attributable to paternal age."
He could not rule out a biological difference in younger men which might be contributing, but said there were more obvious societal reasons.
"Young fathers are more likely to come from economically disadvantages families and to have lower educational attainment," he said.
"People from less affluent backgrounds are less likely to utilise prenatal care services."
Dr Susan Bewley, a consultant obstetrician and researcher at Kings College London, said social factors were more likely to be the reason.
"The question is how good a father can you be when you are a very young person? It's far harder to be a good provider when you are that age.
"It's also possible that a younger father will not offer the same kind of emotional support to a mother."
Dr Allan Pacey, Senior Lecturer in Andrology at the University of Sheffield and Secretary of the British Fertility Society, said that anything suggesting that younger men's sperm was causing the problem "bucked the trend" of many other studies.
"A far more convincing explanation for the finding in this study is that older men are simply better able to provide for their pregnant partners than younger fathers.
"It makes sense that babies born to older fathers probably have a better start to life."