Rates of miscarriage during the first trimester have been reported to be significantly higher in women with PCOS than in those without the syndrome, however obesity is prevalent in women with PCOS and might be a major factor in increasing the risk of spontaneous abortion in these women.

Studies on the effects of metformin on pregnancy loss in women with PCOS vary with regard to the length and timing of metformin treatment.  A Cochrane review concluded that compared to placebo or no treatment, metformin did not affect miscarriage rates in women with PCOS. This review also concluded that metformin did not differ statistically significantly from clomifene with regard to its effect on miscarriage rates. In the studies on which this conclusion was based, metformin was discontinued when pregnancy tests were found to be positive. In addition, a metaanalysis of seventeen randomised, controlled trials found that, when given preconceptionally, metformin, alone or in combination, had no effect on the rate of miscarriage in pregnant women with PCOS. 

Other studies have suggested that metformin has a protective effect on pregnancy in women with PCOS when used throughout pregnancy or during the first trimester. One of these studies included only women with hyperinsulinaemia and it has been advised that metformin use for PCOS be restricted to women with glucose intolerance. In addition, outside of a clinical research setting, it is currently recommended that metformin be stopped on confirmation of pregnancy.

Updated NICE guidelines on fertility are due to be released in July 2012 and are expected to provide advice relevant to this issue.

 
It should be noted that metformin is licensed only for the treatment of type 2 diabetes. Therefore the prescriber takes full responsibility when prescribing metformin for PCOS.