New mothers who deliver after they are 35 or more, run a double risk of suffering postpartum psychosis as compared to younger mums, which could pose a risk to the new born infant.
"The risk of developing psychosis during the first 90 days (after childbirth) increased with age," said the study, conducted by researchers at the Karolinska Institute in Stockholm and published by the Public Library of Science medical journal.
Women over 35 when they gave birth for the first time were 2.4 times more likely to develop postpartum psychosis than those younger than 19, according to the study, which was based on data gathered from all the nearly 750,000 first-time mothers who gave birth in Sweden between 1983 and 2000.
While some 80 percent of new mothers experience some kind of mental disturbance or light depression, only about one in 1,000 women suffer from actual psychosis in the first months after giving birth, the study showed.
Postpartum psychosis is defined as a serious mental disorder, involving delusions, hallucinations, severe eating or sleeping disturbances, suicidal tendencies and can even pose a threat to the newborn baby.
It demands immediate medical attention including the administration of anti-psychotic drugs and hospitalisation.
Most women who suffer from such a psychosis have prior psychotic histories, but the Karolinska research indicated nearly 50 percent of the cases appeared in women "without prior psychotic hospitalisation."
The study meanwhile showed that factors including smoking and not living with the baby's father had no impact on whether a woman developed postpartum psychosis, while suffering from diabetes and giving birth to babies with high birth weights reduced the risk.