Experts said GPs should more closely monitor expectant mothers with the condition for complications.
It is known PCOS increases the chance of complications with pregnancy but studies have not clearly shown that this risk is distinct from the effects of fertility treatment, which many affected women undergo.
The Swedish research compared records from 3,787 births among women with PCOS and 1.1 million among women without.
Women with PCOS were 45% more likely to develop pre-eclampsia and more than twice as likely to have gestational diabetes. Risk of very pre-term birth was 2.2-fold higher.
Infants born to PCOS mothers also had larger than average birth weight and more likely to develop asphyxia during labour.
The heightened risks could not be explained by the use of assisted reproduction technologies.
Researchers concluded: ‘These women may need increased surveillance during pregnancy and childbirth. Future research would benefit from focusing on glucose control, medical treatment and hormonal status among women with PCOS during pregnancy.’
Professor Nick Macklon of the University of Southampton, wrote in an editorial: ‘It is clear that women with PCOS should be considered "high risk" obstetric patients and that midwives, GPs, and obstetricians should monitor these women as such.’