And more women are depressed when their child turns four than at any time before that, according to the study, which challenges the notion that mothers' birth-related mental struggles usually happen at or after the baby's arrival, the Guardian reported.
The findings have led to calls for all women giving birth in the UK to have their mental health monitored until their child turns five to ensure that more of those experiencing difficulties are identified.
The results are based on research in Australia, but experts believe that about the same number of women in the UK experience bouts of mental ill-health associated with becoming a mother.
In all 1,507 women from six hospitals in Melbourne, Australia, told researchers from the Murdoch children's research institute and royal children's hospital in Parkville, Victoria, about their experience of episodes of poor mental health at regular intervals until their child turned four.
The authors found that almost one in three first-time mothers reported "depressive symptoms on at least one occasion from early pregnancy to four years postpartum [and that] the prevalence of depressive symptoms was highest at four years postpartum".
The women's depressive symptoms are often short-lived episodes and do not mean that the women were diagnosed with postnatal depression.
Studies in both the UK and internationally have estimated that between 10 percent and 15 percent of new mothers suffer from that clinical condition.
The researchers also found that four years after the child's birth 14.5 percent display depressive symptoms, of whom 40 percent had not previously reported feeling very low. At that time, women with only one child were much more likely (22.9 percent) than those with two or more offspring (11.3 percent) to be depressed.
The study is published in BJOG: An international journal of obstetrics and gynaecology.