A study of thousands of mothers of children aged five and under found depression rates to be far higher in those aged 40 to 44 than among those who were younger.
Though the cause was unclear, anxiety in pregnancy could be a large factor, according to the researcher behind the study.
Depression rates have been found to be far higher in mothers aged 40 to 44 than among those who were younger
Older mothers and their babies are at a risk of a host of health problems and anxiety in pregnancy is known to raise the odds of depression afterwards.
Giulia Muraca, a PhD student at Canada’s University of British Columbia, studied almost 8,000 women who had given birth in the previous five years.
They were asked if they had been depressed over the past 12 months.
The group of women who gave birth while in their late 30s and early 40s were five times as likely to have been depressed than the mothers with similar lifestyles who were five years younger when they had their baby.
Miss Muraca also looked at women who hadn’t had babies in the past five years and found that age appeared to cut their odds of depression.
This may be because age tends to bring with it more stability in both finances and relationships.
But for women who have children when they are aged 35-plus, these benefits may be cancelled out by health fears, the American Association for the Advancement of Science’s annual conference heard.
Worries: Having a children at an older age can affect a woman's mental stability
Older mothers run a greater risk of problem pregnancies, miscarriage and stillbirths.
Deterioration of their eggs with time also leaves them at greater risk of having babies with Down’s syndrome and other genetic disorders.
And their age makes them more prone to arthritis, depression, cancer and heart attacks as they bring up their children.
Postponing motherhood also affects partners – men’s sperm counts deteriorate gradually each year and children of older men have an increased risk of schizophrenia.
'Older mothers may also suffer from loneliness and a lack of support, if most of their friends had their children years earlier'
Doctors say the optimum age for giving birth starts at 20 and ends at 35.
Despite the risks, though, more and more women are delaying motherhood.
The number of babies born to women aged 35 to 39 in England and Wales more than doubled between 1990 and 2010, to 115,841 a year.
Births to women aged 40 to 44 almost trebled, official statistics show.
Miss Muraca stressed that her work is preliminary and the psychological risks of late motherhood need more research, but added: ‘There is a lot of rhetoric talking about all the biological risk and that is really discomforting for women.’
Older mothers may also suffer from loneliness and a lack of support, if most of their friends had their children years earlier.
The researcher said that another possibility is that women who have spent years building their career may struggle to cope with giving up their job, or find the adjustment of returning to work after having their baby hard to take.
Miss Muraca added that if her findings are confirmed by other studies, it could lead to older mothers being monitored for signs of depression to catch any problems earlier.