OBJECTIVE: To study the risk of adverse pregnancy outcomes in women with polycystic ovary syndrome (PCOS), and to examine the role of hyperandrogenaemia.

DESIGN: Cohort study.

SETTING: Singleton pregnancies in women with PCOS identified at a private fertility clinic during 1997-2010 and a background population including all singleton deliveries at Hvidovre Hospital, Denmark, in 2005.

POPULATION: A cohort of 459 women with PCOS and a background population of 5409 women.

METHODS: Obstetric outcomes were extracted from national Danish registries and odds ratios (ORs) were calculated by multiple logistic regression analysis, adjusting for age, parity, and body mass index.

MAIN OUTCOME MEASURES: Risk of pre-eclampsia, preterm delivery, and small for gestational age offspring in the entire PCOS population and in a subsample with hyperandrogenaemia.

RESULTS: Women with PCOS had an increased risk of preterm delivery <37 weeks of gestation (OR 2.28; 95% confidence interval, 95% CI, 1.51-3.45; P < 0.0001). The elevated risk was confined to hyperandrogenic women with PCOS: preterm delivery before 37 weeks of gestation (OR 2.78; 95% CI 1.62-4.77; P < 0.0001), and was not seen in normoandrogenic women with PCOS (OR 1.35; 95% CI 0.54-3.39; P = 0.52). The overall risk of pre-eclampsia was not elevated (OR 1.69; 95% CI 0.99-2.88; P = 0.05) compared with the background population, but was significantly increased in the hyperandrogenic subsample (OR 2.41; 95% CI 1.26-4.58; P < 0.001). The risk of small for gestational age offspring was similar in all groups.

CONCLUSION: Women with PCOS had an increased risk of preterm delivery compared with the background population. The increased risk was confined to hyperandrogenic women with PCOS who had a two-fold increased risk of preterm delivery and pre-eclampsia.