I understand that Clomid works with the body's own oestrogen but then also works against it too - and yes, I have read that it can thin the lining - so can the BCP. My FS told me that the way Clomid works with the body to induce O is not yet fully understood.

I am not an expert on Clomid because I have never taken it but someone else might know more.

My understanding was that proper O thickens the lining due to the action of progesterone. I believe some Drs actually recommend Clomid to induce proper O and therefore increase progesterone to help prepare the endometrium.