David Cameron says that developing family planning strategies is "fundamental" to tackling global poverty.
The Prime Minister made his comments as the Government pledged to donate more than £1 billion to help family planning services in the developing world.
In a bid to help 24 million girls and women in the world's poorest countries, British aid will be doubled for eight years.
Cash donated to family planning services will increase from £90 million each year to £180 million - more than £1.4 billion in total.
Speaking to health leaders from around the world at the London Family Planning Summit, Mr Cameron said: "Women should be able to decide freely and for themselves whether, when and how many children they have.