What is the difference between low hormone and high hormone birth control pills?

Estrogen dosage. The original pill contained 150ug of estrogen. Over the last 50'years, the dosages have dropped to 35 and 30 ug - relatively higher dosages - to 20 ug - low dosage - to a newer 10 ug pill - the lowest available. Benefits can include shorter, lighter periods and possibly improved safety and effectiveness - with increasing the days taken from 21 to 26.