Population has become a dirty word.
It is a word that many of my predecessors and counterparts have, some might say understandably, steered clear of for decades. That's because it is normally followed by words like 'control' and 'explosion'. It conjures images of forced euthanasia and sterilisation at one extreme or famine, poverty, disease and war at the other.
But as the world's population reaches seven billion - with the number of babies born each month equal to the population of Portugal - now is the time to start talking about it.
Some people will say we're heading for a vast human tragedy if growth continues at the current rate. Others will say we'll cope and that the rising population is simply proof of increasingly sophisticated developments in medicine, agriculture and technology.
It is not for me - or anyone - to tell you how many children to have as that is your choice. And providing choice for women has been my priority since starting this job. In fact, one of my first acts as Development Secretary was to authorise a shipment of condoms to Uganda.
This is partly because lack of choice is such a significant factor in rapid population growth. Millions of unintended pregnancies occur every year; yet at the same time 215 million women who desperately want to delay or avoid pregnancy are unable to do so.
When I was at a health centre in Rwanda, women shed tears of joy and cheered when they discovered they weren't expecting because, for some of these women, pregnancy can amount to a death sentence.
A woman dies in pregnancy or childbirth every two minutes - 99% of them in the developing world. Most of those who do survive are left trapped in poverty, ahead of them a lifetime of struggling to provide food, water, healthcare and education for their children.
For the many millions of girls who are still children themselves, pregnancy inevitably means giving up school and any chance of the education that would allow them to support themselves and their family in years to come. This pattern will continue unless action is taken to provide universal access to reproductive health and family planning services.
That's why over the next four years, British support will make it possible for at least ten million more women to use modern methods of family planning so that they can decide for themselves whether, when and how many children to have. We will also help to prevent more than five million unintended pregnancies and to save the lives of at least 50,000 women during pregnancy and childbirth and a quarter of a million newborn babies.
As a result of these measures, fewer girls will have to leave school because they're pregnant, something that has significant health implications in itself: a girl who has completed her education typically marries four years later and has two fewer children.
In short, family planning is excellent value for money. The relevant services, including the provision of contraception, cost on average under £1 per person per year - far less than treating the complications of an unintended pregnancy or investing in additional services to provide health care and education for a burgeoning population.
The Government of Tanzania, for example, estimates that it will need 131,000 fewer teachers by 2035 if fertility declines - saving millions of pounds in the long run.
Giving girls and women the choice to decide whether, when and how many children they have means fewer women die in childbirth and the poorest families can make what little they have go further.
Family planning is a smart, simple and extremely cost effective investment of aid. It is at the centre of all our development work and we are going to ensure more women are given the choices they rightly deserve.
UK aid is helping the world's poorest people to change their lives by enabling at least ten million more women to use modern family planning and prevent five million unintended pregnancies. Helping couples plan their families means they can give their children a better chance in life.
Find out more about what DFID is doing, in our feature on 'A World at 7 Billion'
This article first appeared on the Huffington Post website.