Depressed fathers-to-be are more likely to have a child born prematurely, a study has found.
Scientists have long known that depression among pregnant women increases the chance of a premature birth.
But a major study has now established that depression among men also makes a preterm birth more likely.
Researchers tracked more than 350,000 births for five years - and found that depression has a major impact on a birth, for both mothers and fathers.
Men who became depressed in the 12 months before their partner conceived, or in the first six months of the pregnancy itself, had a 38 per cent increased chance of their child being born 'very' prematurely, defined as arriving before 31 weeks.
Depressed fathers also had a 12 per cent increased chance of their partner giving birth 'moderately' prematurely - between 32 and 36 weeks.
Mothers who had become depressed had a 23 per cent increased chance of having a very premature baby and 34 per cent of a moderately premature baby.
The Swedish researchers, from Stockholm University, suspect that a father's mental health has a profound impact because it, in turn, places stress on the mother.
An alternative theory is that depression may decrease the quality of a man's sperm.
Research author Professor Anders Hjern, whose work is published in the British Journal of Obstetrics and Gynaecology, said: 'Depression of a partner can be considered to be a substantial source of stress for an expectant mother, and this may result in the increased risk of very preterm birth seen in our study.
'Paternal depression is also known to affect sperm quality, have epigenetic effects on the DNA of the baby, and can also affect placenta function.
'However, this risk seems to be reduced for recurrent paternal depression, indicating that perhaps treatment for the depression reduces the risk of preterm birth.'
He added: 'Our results suggest that both maternal and paternal depression should be considered in preterm birth prevention strategies and both parents should be screened for mental health problems.
'Since men are less likely to seek professional help for any mental health problems, a proactive approach towards targeting the wellbeing of expectant fathers may be beneficial.'
Dr Patrick O'Brien, spokesman for the Royal College of Obstetricians and Gynaecologists, said last night: 'Depression in pregnancy can be very serious for a woman and can also impact on the health of her baby.
'We know that between 12 per cent and 20 per cent of women experience anxiety and/or depression during pregnancy and the first year after childbirth.
'This research is interesting as it finds that paternal mental health can also have an effect on the health of the baby. However, more research is needed to establish the mechanism behind this effect.
'We encourage anyone, and particularly those planning a family or who are pregnant, and are experiencing a change in mood, irritability or anxiety to seek advice. No-one should suffer in silence - there is help and support available.'
John Thorp, deputy editor-in-chief of the British Journal of Obstetrics and Gynaecology, said: 'This study highlights the importance of treating depression for both men and women, and the impact untreated depression can have on the health of offspring.
'Further progress is needed into the understanding of how depression of either parent affects pregnancy in order to help prevent preterm birth.'