Melinda Gates, wife of the Microsoft magnate Bill, flew in from a field visit to Niger and Senegal on Tuesday, and will have risen by 4.30 yesterday morning to conduct meetings and interviews before the real working day begins.

In her capacity as co-chairman of the Bill & Melinda Gates Foundation, she will then join Andrew Mitchell, Secretary of State for International Development, in hosting a family-planning summit for global leaders in London. Together, they will launch a $4 billion fund-raising effort that would deliver safe contraception to 120 million women and girls in developing countries.

"All lives have an equal value," says Gates simply. "Unwanted and unplanned pregnancies result in more than 200,000 women and girls dying in pregnancy and childbirth, and nearly three million babies dying in their first year of life. That's a shocking situation - but one that we can change."

Gates, 47, is impeccably, if anonymously, dressed in a grey trouser suit, her cascade of hair blow-dried just so. She must be exhausted, but it doesn't show. Bill is at home with their daughters Jennifer, 16, and Phoebe, 10, and son Rory, 13. They try to ensure that globetrotting doesn't interfere too much with family life, and since she and Bill started working for their foundation full time, it's been easier to harmonise their schedules and share the childcare.

"I'm wholehearted about whatever I do," says Gates with a wry smile. "So every year I meet with the kids' new teachers and tell them to let me know if I'm being a helicopter mom, because I really don't want to be. And yes, they've had the nerve to say, on occasion, 'I think we're all a little over-focused on this issue,' which gives me the message!"

It's hard, here in Britain, to grasp that philanthropy qualifies as a day job. After all, less than two months ago the Government sought to remove tax breaks for charitable donations from the wealthy -before executing a rapid U-turn in the face of outrage from charities.

But Gates and her husband control a foundation worth almost $34 billion, a sum higher than the GDP of many countries. She's been dubbed joint president of the United Gates of America, which makes the pre-dawn breakfast talks and 10pm strategy discussions easier to understand. But why the endless legwork? Why not just write a cheque, smile for the cameras, hand it over to an existing charity and still make the last act of La Traviata?

"We're not giving out aid, we're providing people with the tools they need to improve their lives; seeds they can sow, access to better health care. And to be effective you have to understand the issues at a very deep level."

While her husband's interest lies in the power of innovation and science to develop vaccines for HIV/Aids and malaria, Gates is engaged in trying to give women more control over their health. In male-dominated cultures, children are seen as an index of a father's virility - with little thought to whether a mother can feed them all.

"It's heartbreaking to have a mother thrust her baby at you and beg you to take him home because otherwise he's going to die," she says quietly.

The philanthropist Andrew Carnegie famously said that "to die rich is to die disgraced". The Gateses could have lavished their wealth on baubles: symphony orchestras, cultural institutions or architecturally daring art galleries. Instead, they are determined to fund Cinderella causes.

"I care much more about saving the lives of mothers and babies than I do about a fancy museum somewhere," she says. "Knowing you've got a family started on a path to self-sufficiency, that is job satisfaction. There's a false perception that women in Africa somehow don't love their babies they way we do, don't grieve their loss the way we would. That is simply not true. These women are as heartbroken as any of us would be, and that's why they need access to contraception - so they can breastfeed each baby and give him or her a good start in life, without becoming pregnant again immediately.

"I once met a woman in a slum outside Nairobi who said, 'I want to give every good thing to this child before I have another one.' Isn't that what we all want?"

There's something downright contrary - admirably so - in the couple's insistence on turning the world's reluctant gaze on the ugly affliction of Aids and HIV, on the unglamorous problems of maternal death and infant mortality. These issues may be about sex, but they are most definitely not sexy.

The Gates foundation has pumped money into the American education system, and global development in the form of microfinance, agriculture and world health. The unflinching principles of business are at the core of their charitable enterprise. If the results aren't as good as they should be, at best they review their strategy and at worst they won't reinvest. Maximum effectiveness is their bottom line.

Bill's close friend, the billionaire investor Warren Buffett, has already signed over a tranche of cash to the foundation, with an added proviso that after his death, it spends his entire wealth, currently estimated at $44.1 billion, within a decade.

Gates's conscience was cultivated from an early age. Unlike her Harvard drop-out husband, who was born into a privileged Seattle background, she is one of four children brought up in modest circumstances in Texas, where education was regarded as the holy grail. Her housewife mother regretted not attending college. Her engineer father set up a cleaning business on the side to raise the cash for his children's education, and as a teenager Gates scrubbed floors to help out.

She and her siblings were also encouraged to volunteer in charity projects. Hard-working and bright, she graduated top of her year at the Ursuline high school she attended.

Her Catholicism has led Christian conservatives to criticise her advocacy of birth control. "So many people try to polarise the issue," she says. "A total of 82 per cent of Catholics in the US believe contraception is morally acceptable, and I can see what a massive benefit it represents for women struggling to give their children opportunities that will lift them out of poverty."

Gates was a computer science student at Duke University in North Carolina before she joined Microsoft, where she was involved with the development of products such as Encarta and Expedia. Then she began a relationship with the boss, but denies the apocryphal story that they communicated with each other via Post-it notes. Would it be fair, however, to call him a geek? "Oh yes, but he prefers the term 'nerdy'. He's the very definition of it," she laughs.

She married Bill in Hawaii in 1994, and left the company to bring up their family at their mansion on Lake Washington. Having vowed to give 95 per cent of their wealth (approximately $58 billion) back to society, the Gates children will have, as Warren Buffett so neatly puts it: "enough to do anything, but not enough to do nothing".

But surely there is some room for relaxation in the Gates household? "My ultimate luxury is a Friday night at home watching a DVD with a bowl of not-too-salty popcorn," she says. Presumably the only difference between Them and Us being that they have that custom-built cinema.

"We installed 'love seats', which wasn't such a good idea," she says. "So there are endless discussions about who gets to sit with whom, where does the dog go and just how many people you can comfortably fit into one large seat - the answer being that with a little planning, a lot more than you'd think."

Perhaps, thanks to the ongoing efforts of the Bill & Melinda Gates Foundation, the same could be said for the planet.

- The Telegraph, London