Save the Children released the report 'Every Last Girl' for the International Day of the Girl Child on Tuesday.
"While there is much to celebrate, there is still a mountain to climb until we reach a world in which girls will have the same opportunities as boys," wrote former Danish PM Helle Thorning-Schmidt, now the CEO of Save the Children International, and Kevin Watkins, CEO of Save the Children UK, in a press release.
The report used its so-called Girls' Opportunity Index to rank the world's countries according to five indicators: child marriage, adolescent fertility, maternal mortality (as an indicator to access to good-quality healthcare), women MPs and lower-secondary school completion.
It ranked Sweden at the top, followed by Finland, Norway, the Netherlands, Belgium and Denmark.
Some other developed nations such as the UK (15), Canada (19) and the United States (32) were pulled down the rankings by not having enough women represented in government.
Thirty-seven percent of Danish MPs are women, compared to 29 percent of lawmakers in the UK and 19 percent in the US. Sweden had the highest proportion with 44 percent women.
"Only three of the countries with the highest proportion of female MPs are high income countries – Sweden, Finland and Spain. Rwanda tops the table with 64 percent of female MPs, followed by Bolivia and Cuba," read the report.
The report also pointed out the US's relatively high adolescent fertility and maternal mortality rates. Fourteen women died per 100,000 live births in the US in 2015; a similar number to Uruguay and Lebanon, and far higher than the three deaths per 100,000 in Poland, Greece and Finland," the report stated.
At the bottom of the list were Niger (144), Chad (143), the Central African Republic (142), Mali (141) and Somalia (140).
"The worst places to be a girl are amongst the poorest in the world. These countries have extremely high rates of deprivation across all indicators. They must focus urgently on ensuring that policy and practice uphold girls' rights," said the report.
While Denmark was ranked sixth in the Every Last Girl study, rankings released by the US News & World Report in January had Denmark as the best country in the world for women.
Thorning-Schmidt took on the role of Save the Children CEO in April. She was Denmark's first ever female prime minister, serving from 2011 to 2015.