Hi CrispyChick, I'm not an 'HRT expert', all I do is read the available info and try to understand it, so what I'm thinking is that Zoely's action relies on the 2.5mg nomegestrol acetate by inhibiting ovulation and changing cervical secretion, according to the SmPC for the drug 'The contraceptive effect of Zoely is based on the interaction of various factors, the most important of which are seen as the inhibition of ovulation and the changes in the cervical secretion' Don't ask me what 'various factors' means...