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
The amount of estrogen in low dose oral contraceptive pills is: 
Select and explain. 
A. 30 micrograms, B. 40 micrograms, C. 50 micrograms, D. 20 micrograms
D: 20 micrograms
Answer: D: 20 micrograms 
Explanation: Over time, the estrogen and progestin contents of COCs have been reduced remarkably to minimize hormone-related adverse effects. Currently, the lowest acceptable dose is limited by their ability to prevent pregnancy and unacceptable breakthrough bleeding. Although daily estrogen content varies from 20 to 50 g of ethinyl estradiol, most contain 35 ug or less. Low-dose COCs usually contains 20 ug ethinyl estradiol. Ref: Leveno K.J., Hauth J.C., Rouse D.J., Spong C.Y. (2010). Chapter 32. Contraception. In K.J. Leveno, J.C. Hauth, D.J. Rouse, C.Y. Spong (Eds), Williams Obstetrics, 23e.