The appointment on Tuesday of two Democrats makes the Nevada state legislature the first in the country where women hold a majority.

Beatrice Duran and Rochelle Thuy Nguyen were selected by the Clark County Board of County Commissioners for recently vacated Las Vegas-area seats that they will hold until the next general election in 2020.

Starting in February, women will hold 50.8 percent of the state's 63 legislative seats – 23 in the Assembly and nine in the Senate. No state has ever had a female majority, or even surpassed the 50 percent mark, according to the Center for American Women and Politics at Rutgers University.

"It's worth celebrating that we have finally have one state legislature that more accurately reflects the population it serves, in terms of gender diversity," Kelly Dittmar, a political scientist at Rutgers University, told CNBC Make It. "But as with most political milestones for women, it reflects progress and also serves as a reminder about how the gap in women representation persists."

Across America, the number of women in legislative seats is on the rise after remaining stagnant for nearly two decades. In 2019, women-held seats will grow to at least 28.6 percent in state legislatures, up from the current level of 25.4 percent, according to data from Rutgers. The proportion of women in state legislatures has historically remained between 23 and 25 percent.

Women also made gains in specific chambers across the U.S. Before 2018, the only state legislative chamber to have a female majority was in the New Hampshire Senate in 2009. In addition to the legislative majority in Nevada, women now make up more than 50 percent of the Colorado House.

Women will also make up more than 40 percent of state legislators in Colorado, Nevada, Oregon and Washington, and eleven states will see women's legislative representation increase to over 5 percent, according to Rutgers data.