Prognostic factors influencing fresh and frozen IVF outcomes: an analysis of the UK national database

National registry data provides a valuable resource which can be used to determine to what extent clinical and patient characteristics influence the outcomes of IVF treatments. We fitted complex statistical models for live birth and multiple births to data from the UK national registry of treatments conducted between 2000 and 2005. The analysis included 119,930 fresh and 19,918 frozen transfers from 85,349 patients in 84 centres. As well as quantifying the effects of a range of previously identified prognostic factors, we were able to estimate the magnitude of the losses due to embryo freezing and thawing. The effects of clinical and patient characteristics on the outcomes for frozen transfers were largely similar to those for fresh cycles. No factors were identified which specifically predicted twin outcomes; patients with a high twin risk were those with a high chance of success. After allowing for all the identified prognostic factors, there remained clinically important variability between centres and between patients which may suggest that the patients and clinics differ in their chances of success due to characteristics which are not currently being measured.