steroid hormone biosynthetic pathway C18-steroid hormone biosynthetic pathway 2010-05-20T02:42:49Z Estrogen biosynthesis PW:0000780 Those metabolic reactions involved in the synthesis of C18-steroid hormones. Pregnenolone, a C21 steroid derived from cholesterol, and progesterone, to which pregnenolone can be converted, provide the starting material for the C21, C19 and C18 steroid hormones. The C18 class is represented by estrogens. The major natural estrogens are beta-estradiol - the primary estrogen, the estrone of menopause and the estriol of pregnancy. VPetri estrogen biosynthetic pathway estrogen biosynthetic process pathway