08/10/2013 09:51 BST | Updated 08/12/2013 05:12 GMT

Girls Draw the Short Straw in Life - We Must Help Them Change This

Did you know that more than 140million girls will become child brides by 2020 if current rates continue? That's 39,000 girls married off every day.

It's a shocking statistic, and no less shocking every time I quote it to policy makers, celebrities and politicians as I lobby for them to join Plan International in our campaign for the futures of girls around the world.

Today is International Day of the Girl, the second ever of its kind and one that I believe is another important milestone in our fight to improve the lives of girls around the globe and help them fulfil their potential.

Girls in the developing world tend to draw the short straw in life. They are intrinsically vulnerable, and face everything from the threat of early marriage and violence to the simple fact that their parents do not think girls are important enough to go to school.

Recently I travelled to South Sudan, where I met 10 year-old Martha. "My father says I'm very beautiful," she told me, explaining why she had to drop out of school last year due to the drought. "I'll bring him many cows when he marries me off. He says I shouldn't bother with education - just stay home and wait to be married."

Martha made me think of my own daughters, now grown-up, and how little they could have achieved without the education they had. For them, school was a given, but girls like Martha have little chance of escaping the cycle of poverty, discrimination and exploitation in which they find themselves.

Globally, one in five girls like Martha is denied a secondary education, and girls' primary school completion rates are below 50 per cent in most poor countries.

The odds really are stacked against girls in many parts of the world, particularly those from the poorest and most marginalised communities.

Yet research shows that when girls reach their full potential, through improved status, better healthcare and education, it is an incredibly effective development tool for society as a whole.

Each extra year of a mother's schooling cuts infant mortality by between five and 10 per cent. It has been estimated that universal secondary education for girls in sub-Saharan Africa could save as many as 1.8million lives annually.

An extra year of secondary school increases a girl's potential income by 15 to 25%. An increase of only one per cent in girls secondary education attendance, adds 0.3% to a country's GDP.

I call on governments and policy makers to help us give every girl a quality education. We want the issue of child marriage raised in the Human Rights Council and we want to pursue a General Assembly Resolution which addresses child marriage as a violation of children's rights. Because a girl who makes it through both high quality primary and secondary education is less likely to experience violence or marry and have children whilst she is still a child, and more likely to be literate, healthy, survive into adulthood (as well as her children), invest her income into her family, community and country, understand her rights and be a force for change.

We know that supporting girls' education is one of the single best investments we can make to help end poverty.

It saves lives. It transforms futures. It unleashes the incredible potential of girls and their communities.

And we know that when girls reach their full potential, through improved status, better healthcare and education, it is the most effective development tool for society as a whole.

So let's put girls into school - and let every girl in the world achieve her true potential.