This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Gynaecology & Obstetrics
Naturally occuring estrogens are:a) Estroneb) Estrad iolc) Estriold) Diethylstilbesterole) Pregnanediol 
Select and explain. 
A. ab, B. bc, C. abc, D. bcd
C: abc
Answer: C: abc 
Explanation: Natural estrogens are C18 steroids
Main Source - Theca and granulosa cells of Graafian follicle and corpus luteum.
Secondary source - Adrenal cortex


Naturally occurring estrogen


EstradioI – Main estrogen during reproductive years,
Estrone (Produced by peripheral aromatization of androstenedione) - Main estrogen after menopause
Estriol (the main source is placenta) - Main estrogen during pregnancy. It is a marker of feto-maternal-placental well being.