Progesterone and Progestins, What's the Difference?

I suggest that those who still insist that progestins and progesterone are the same, or that progesterone is a generic term that also covers progestins, ponder the following questions. If progesterone and progestins are the same:

• Why do fertility doctors always use progesterone and not progestins?

• Why do progestins cause birth defects, while progesterone is essential for a viable and healthy pregnancy?

• Why don't synthetic progestins show up in blood and saliva tests of progesterone levels? In other words, why doesn't taking progestin raise progesterone levels in the body?

• Pregnant women are making 300 mg of progesterone daily in the last trimester. Why don't they have higher rates of breast cancer, as women do who use progestins?

• Why doesn't natural progesterone cause the side effects listed for medroxyprogesterone acetate (Provera) the most commonly used synthetic progestin for HRT?