liar liar pants on fire
If a woman has not ovulated, the morning B pill will stop ovulation. That is why we use it if we see a woman within hours after a rape. But if you give it later, it will stop the fertilized egg from implanting in the uterus, and if she is a few days pregnant, it will starve the growing fetus (actually at this stage an embryo) and cause an early abortion.
As for "medical definition": Actually until a few years ago, a pregnancy started with fertilization. The pro aborts in the OB GYN societies "changed" the definition, partly so that the 100 thousand fertilized eggs/early embryos sitting in fridges because they were left over from IVF ('test tube babies') wouldn't be called pregnancies...
It is immoral as far as Catholics are concern that any contraception is wrong. It violates the gift of life