Progesterone, injected into the body, can mimic a pregnancy and so stop ovulation. However, injection is not a practically solution to birth control, and taken orally it was ineffective.
Norethindrone is one of a number of chemicals that have the same effect in the body as progesterone and which are effective when taken orally.
Structurally, note that norethindrone lacks one of the methyl groups of progesterone, and the substituent COCH3 has been replaced by an OH group and an acetylene group.
Birth control pills consist of mixtures of an estrogen (to control the menstrual cycle0 and a progestin (such as norethindrone) in varying amounts.
Initiates a spontaneous abortion, and prevents a fertilized egg from implantation in the uterus wall thereby preventing pregnancy after fertilization has taken place. Since it is taken after the impregnation it is sometimes refered to as the 'Morning after' pill.
Controversy reigns in the US due to the chemical being used to initiate abortions after attachment to the uterus wall has occured, thereby replacing a surgical procedure.
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