Babies are more likely to be born premature if either parent suffers depression around the time of pregnancy, research suggests.
While experts know that depression in the mother can lead to early labour and low birth weight babies, little is known about the effects of depression in men.
Now a study published in BJOG: an International Journal of Obstetrics and Gynaecology has found that either parent can affect prematurity.
Experts in Sweden studied more than 350,000 births between 2007 and 2012 and looked at very premature births (between 22 and 31 weeks of pregnancy) or moderately premature births (32 to 36 weeks).
For both men and women, depression was defined as having had a prescription of anti-depressant medication, or receiving outpatient or inpatient hospital care in the 12 months before conception to the end of the second trimester of pregnancy.
Experts found new and recurrent cases of depression in women increased the risk of moderately premature birth by 30% to 40%.
Meanwhile, new cases of depression in men (defined as suffering no previous depression in the 12 months before being diagnosed) was associated with a 38% increased risk of very premature birth. Recurrent depression in fathers was not associated with premature birth at all.
Professor Anders Hjern, from the Centre for Health Equity Studies in Stockholm, 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.
"For the mothers, depression increased the risk of preterm birth, regardless of whether the depression was new or recurrent."
Dr Patrick O'Brien, an obstetrician and spokesman for the Royal College of Obstetricians and Gynaecologists (RCOG), said: "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% and 20% 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."