Health professionals need to be vigilant for depression in pregnancy
Women who are depressed during pregnancy can have babies who develop more slowly than their peers, a UK study suggests.
Postnatal depression is known to cause this, but the researchers say antenatal depression can have its own impact.
Writing in the BJOG journal, they said it could mean a third greater chance of cognitive or behavioural problems.
Midwives are trained to spot depression, and report significant problems to GPs.
The study looked at the records of 11,098 women and their children who gave birth in 1991 and 1992.
They assessed the level of depression shown by women during pregnancy, then looked for a relationship between this and any developmental problems in their children.
Women with persistent depression during pregnancy were 50% more likely to have children with diagnosed problems.
However, some of that risk comes from the fact that being depressed during pregnancy boosts the chances of postnatal depression, a known risk factor for developmental delay in children.
The scientists worked out, however, that a 34% rise in risk could be linked independently to antenatal depression, and nothing else.
It is not clear exactly how depression before, rather than after, the birth can have an impact on child development, although one study has suggested that women who are depressed prior to birth may be more likely to give birth prematurely.
Dr Toity Deave, from the Centre for Child and Adolescent Health at the University of the West of England, said: "The most important finding is that maternal antenatal depression has a negative impact on children's cognitive development, even when postnatal depression has been taken into account."
Professor Philip Steer, the editor of the BJOG journal, said that it was "essential" for doctors and midwives to play an active role in assessing and identifying the condition.
A spokesman for the Royal College of Midwives said that, by building relationships with pregnant women, midwives were well-placed to do that.
She said: "There is still a stigma attached to depression and mental illness in pregnancy, and sometimes it can help just to acknowledge there is a problem.
"Where the problem is more serious, midwives are trained to refer on to a GP or community psychiatric nurse so that a woman gets the support and help she needs."