How should recurrent implantation failure be approached in women with PCOS?

[Terminology: PCOS was renamed polyendocrine metabolic ovarian syndrome (PMOS) by global consensus in May 2026, published in The Lancet. The International Guideline now uses PMOS/PCOS in its recommendations and will use PMOS alone from 2028. Clinical content, diagnostic criteria and management are unchanged.]

The general RIF approach applies, with several PCOS-specific considerations layered on. Metabolic factors. Insulin resistance and obesity are associated with impaired implantation independent of embryo quality. Weight reduction where relevant improves outcomes, and this remains the intervention with the best evidence in this group — though it is slow and should not delay treatment indefinitely in older patients. Endometrial factors. Chronic anovulation means prolonged unopposed oestrogen exposure, and endometrial hyperplasia should be excluded in women with prolonged amenorrhoea before repeated transfers. This is under-considered. Cycle type. Where OHSS risk led to freeze-all, endometrial preparation protocol becomes relevant — natural, modified natural, or programmed cycles differ in luteal support requirements, and inadequate luteal support in programmed cycles is a recognised and correctable cause of failure. Embryo factor. PCOS patients frequently produce high oocyte numbers with variable maturity. Reviewing maturation rates and laboratory performance is often more informative than adding endometrial investigations. What not to add reflexively: ESHRE (2026) states routine metformin before or during stimulation is probably not recommended with the antagonist protocol in PCOS (conditional), and growth hormone is not recommended in PCOS (strong). Thyroid function and prolactin should be checked, as both are more prevalent in this group and both are correctable.