How does estrogen in contraception pills prevent fertilisation in the first place?

Works in pituitary. The estrogen in the pill alters the normal feedback to the pituitary gland, which in turn prevents the changes (rise) in fsh (follicle stimulating hormone) and lh (luteinizing hormone) levels which lead to ovulation, which is the release of the egg from the ovary. Since the egg doesn't leave the ovary, it is not physically accessible for the sperm to fertilize it.