The researchers at Karolinska Institutet in Solna found that women who were 35 and above were 2.4 times more likely to suffer from postpartal psychosis in the 90 days after giving birth than first-time mothers aged 19 or younger.
"The risk of psychosis increases sharply in close relation with the birth of a first child for women both with and without a previous psychiatric diagnosis," said one of the researchers, Dr Unnur Valdimarsdóttir, in a statement from KI on Tuesday.
Post-partum psychosis is, in contrast with post-natal depression, relatively unusual, KI writes.
The study compared the data of 750,000 women who gave birth to their first child in Sweden between 1983 and 2000.
Researchers found that 892 of the women in the study (1.2 in every 1,000 births) were hospitalized for a psychotic illness within 90 days of their first delivery. Almost 50 percent, 436 women, became ill for the first time.
Researchers paid particular attention to women with no history of psychiatric illness. Comparisons were made on the basis of several background factors, such as age, education and delivery characteristics.
"We know from previous studies that women who've had a previous psychiatric condition are more likely to develop postpartum psychosis," said Valdimarsdóttir.
"Our intention in this study was to identify factors that increase the risk of postpartum psychosis in women without a history of psychiatric hospitalisation."
High birth weight and maternal diabetes were however also found to correlate with a lower risk of psychosis and Dr Valdimarsdóttir said more studies were needed to better understand the causal factors, such as hormonal changes during labour.
The risk of developing a psychosis declined considerably after the 90 days had passed, the study concluded.