Read the package inserts from the manufacturer. Every hormonal birth control on the market -- EVERY one -- does three things: (1) it may prevent ovulation if taken at the right point in the cycle; (2) it thickens cervical mucus so sperm can't penetrate; and (3) it makes implantation in the uterine lining impossible, causing the already-conceived baby to die.
It’s my feeling that while regular use of hormonal contraception for several months can change the lining of the uterus so that it becomes inhospitable, a one-day spike of progesterone during a natural cycle should not affect the lining that has already developed, especially if taken so close to the time of ovulation that it fails to prevent ovulation, or if taken afterward.
The literature is poor, and if emergency contraception researchers cared about the ethical issues involving a fertilized egg, or cared about the ethical autonomy and responsibility of women taking these pills, it would likely be more definitive.
Yes, but the actual studies done on this have shown that it doesn’t affect implantation at all...so, the implantation claim is weak, at best.