The most significant change in the role of women in the United States occurred in 1920 with the passage of the nineteenth amendment, which gave women the right to vote. In this day and age it is very difficult indeed to imagine that women did not have this right until well into the 20th century.
You see the effect of this change during significant election cycles. Since women comprise a large segment of the voting population, politicians have to take their concerns into account or risk losing their votes. The fact that they have electoral power is at least partly responsible for legislation about equal pay, abortion, and other issues that have a particular effect on women.