BACKGROUND: A high proportion of infertile patients have polycystic ovarian syndrome (PCOS) with a reportedly greater risk of spontaneous abortion. Because of the close link between PCOS and obesity and the independent association of obesity with poor pregnancy outcomes, it is important to distinguish the possible confounding effect of body mass index (BMI) or other variables from that of PCOS. This study aims to determine the effect of PCOS status on the risk of spontaneous abortion with

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... ment for body mass and several other confounding factors in a large cohort of pregnant infertile women. METHODS: The patients (n ⍧ 1018) were treated in a tertiary infertility centre. Their PCOS status was determined by standard criteria and their BMI had been taken less than 1 year before the pregnancy. Patients whose PCOS status or BMI measurements were not assessed were excluded. Student's t-test or χ 2 test were used to test the difference between the PCOS and non-PCOS groups while a multivariate logistical regression model was used to assess the effect of PCOS, BMI and other confounding factors. RESULTS: Overall, the incidence of PCOS was 37% in this cohort. The overall incidence of spontaneous abortion in the study population was 21%. Univariate analysis showed that women with PCOS had a significantly greater risk of spontaneous abortion compared with non-PCOS women (25 versus 18%, P < 0.01). However, using multivariate logistic regression analysis this effect was reduced to a non-significant level [odds ratio (OR) ⍧ 1.10, 95% confidence interval (CI) 0.85-1.36] after adjusting for obesity and patients/treatment combination factor, and to nil after adjusting for all confounding factors considered in this study (OR ⍧ 0.98, 95% CI 0.75-1.28). CONCLUSION: The results of this study suggest that the higher risk of spontaneous abortion observed in women with PCOS is likely to be due to their high prevalence of obesity and the type of treatment they receive.