Girls are more likely than boys to be malnourished, suffer poverty, face violence and be refused an education, according to a new report.
Educating girls is vital to a country's economy, Plan says
Gender inequality and discrimination continues to be pervasive and widespread, Plan International says.
This inequality hinders governments' ability to meet key development goals, the child-based aid organisation says.
The study collated statistics from around the world looking at the lives of girls under the age of 18.
"Girls and young women, who make up almost a quarter of the world's population, probably face the greatest discrimination of any group of this size in the world," the report says.
Girls risk discrimination from the moment they are conceived, Plan International says, facing a far higher chance of being aborted before birth and malnourished or mutilated afterwards in societies which value boys over girls.
Some 121m children do not attend primary school - 54% of those are girls
82m girls in developing countries who are now aged 10-17 will be married before their 18th birthday
In some countries, most girls marry before their 18th birthday: India (50%), Nepal (60%) and Niger (76%)
Worldwide, some 14m 15-to-19-year-olds give birth each year
Girls aged 15-19 are twice as likely to die in childbirth as those in their 20s
Source: Plan report
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Bangladesh, India, Nepal, Pakistan, Algeria, Egypt and Morocco are cited as countries where there is a strong preference for baby boys over girls.
Plan cites estimates that put the number of girls and women who are "missing" - aborted due to their sex before birth - as high as 100 million, despite the biological trend for women to marginally outnumber men.
China alone accounts for 50 million "missing" women, it says.
Economically, the report argues, it makes no sense to neglect girls and young women who have a real and valuable contribution to make to lifting countries out of poverty.
Many societies value boys over girls, seen as a financial burden
Educating girls, Plan says, can transform a country's economy.
"No country has yet emerged from poverty without investing in education, including in that of its girls. Investment in girls bears fruit," the report says.
Education serves to change the lives of girls and young women for future generations too - educated girls are more likely, as mothers, to send their own girls to school.
Women also face health challenges linked to their gender - pregnancy is the leading cause of death for young women aged 15 to 19 and girls are infected by HIV/Aids in disproportionately high rates.
Early marriage in many part of Sub-Saharan Africa and Asia means that many engage in intercourse even before they are teenagers and have limited choice over birth control.
Nearly 50% of sexual assaults worldwide are against girls aged 15 years or younger. Those living in the conflict zones are particularly at risk.
Plan International says that education remains the key to improving the situation for girls and young women and in changing acceptance of discriminatory practices within a society.
It also calls for women under the age of 18 to be listened to and their opinions used to inform policy and decision-making.
Plan says that the legal tools - such as international agreements and human rights charters - already exist but their principles must be followed.
Groups who encourage change, Plan says, should be supported, "to ensure that when a child is born she is not discriminated against simply because she is a girl".