"There is no room for complacency," warned Martin Chungong, head of the IPU, which groups 166 national legislatures.
Women hold 22.1 percent of all parliamentary seats worldwide, up from just 11.3 percent in 1995.
Switzerland, whose president this year is Simonetta Sommaruga, ranked in 34th place with women accounting for 30.5 percent of federal MPs and 19.6 percent of seats in the senate (upper house of parliament).
But while Europe, and especially the Nordic countries, dominated the world's top-ten most female parliaments in 1995, the picture is far different today.
Sub-Saharan African countries currently dominate, with Rwanda tops with 63.8 percent women in its parliament, followed by Bolivia and Andorra, IPU said.
Sweden, which ranks sixth, is meanwhile the only country to have elected more than 40 percent women to every parliament since 1995.
But growth basically ground to a halt last year, inching up just 0.3 percentage points globally after jumping 1.5 points a year earlier.
"After the optimism and belief in 2013 that gender parity in parliament was within reach in a generation, the lack of significant progress in 2014 is a major blow," Chungong said.
"It is a timely reminder that progress is not a given. Political action and will must remain a constant if we are to successfully tackle the gender deficit in politics," he added.
Over the past two decades, social, economic and political transformations have eroded patriarchal values in many societies, thus allowing women to take on different roles, IPU said.
The main driver of increased representation of women in politics has, however, been quota systems, now used in more than 120 countries.
But IPU said the "fast-track impact of quotas" now seems to have reached its peak, and that other measures were needed to broaden women's political participation.
Still, 20 years ago, there was just one parliament in the world where women held more than 40 percent of seats — in Sweden.
Now there are 13.
The number of male-only parliaments in the world has, meanwhile, dropped from ten to five.
For more information, check the IPU website.