The predominant natural estrogens of the human are estradiol- 17β, estrone and estriol. Estrogens contain eighteen carbon atoms. Steroid hormones are derived from cholesterol. The basic structure of all the steroid hormones is the steroid ring cyclopentanoperhydrophenanthrene, having 17 carbon atoms. The estrogens act directly or in cooperation with other hormones to produce a great variety of effects on specific target organs and on the chemistry of the body as a whole.
Figure: (a) cyclopentanoperhydrophenanthrene (b) Estrone (c) Estradiol-l7β (d) Estriol