The amount of estrogen in low dose oral contraceptive pills is:

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.