Many herbs used for these purposes are believed to be teratogens, substances that cause the development of abnormal structures in the embryo. Usually teratogens in a woman's system during the first two weeks of pregnancy (from conception) will cause the pregnancy to terminate.

To help you understand why this is, I've added a section on the day to day development of the fertilized egg with pictures.

Fertility Awareness Section		Preventing and Ending Pregnancy