We have no idea whether it will be in Mumbai, Moscow, Mogadishu or Manchester, but one thing is for certain: next month, the world's seven billionth person will be born. And wherever that baby comes screaming and kicking into the world, it will hopefully bring joy to its parents and enjoy a long and productive life.

Certainly the prospects for Number Seven look brighter than that those predicted for their six billionth cousin (born in 1999), or even the fifth (born in 1987): UNICEF revealed this week that the number of children under five dying each year has declined from 12 million to 7.6 million; good progress towards meeting Millennium Development Goal 4 which aims to see a two thirds reduction in the under 5 death rate by 2015.

Progress - but not enough. On present trends that vital goal will be missed. On the same day that our seven billionth baby is born, 21,000 children under five will also die around the world, mainly from diseases that are easily preventable, and because they don't see a health worker who could help keep them healthy.

Currently there is a shortage of 3.5 million health workers in the world's poorest 49 countries. No matter what progress is made on drugs, vaccines and medical procedures, without health workers, none of these vaccines can be administered, no life-saving drugs prescribed and no expert care given in childbirth. That is why Save the Children and more than 300 organisations around the world are calling on world leaders to take action to close the health worker gap at a high-level event at the UN General Assembly on 20 September.

To deliver basic healthcare, you need at least 23 doctors, nurses and midwives per 10,000 people. Many countries are falling dangerously below this: in Ghana they have only half the number of workers they need, while in Sierra Leone it is less than a tenth. Yet a child is five times more likely to survive to its fifth birthday if they live in a country with enough health workers.

Often it's not just that there aren't enough health workers, but also that they aren't in the right places. For example, 46% of South Africans live in rural areas, but with only 12% of doctors there to serve them. Health workers may need better resources or training to be able to do their jobs to the best of their ability. Or they may be paid so poorly themselves - UNICEF estimates that in one in five countries, nurses barely earn enough to keep them out of poverty - that they end up taking up second jobs or charging patients for care, with the result that the poorest families cannot afford for their children to be treated.

But by tackling the shortage of health workers, a big difference can be made. Malawi's Emergency Human Resources Programme (EHRP) increased the workforce by 53% between 2004-9 and has saved an estimated 13,000 lives. In Nigeria, a midwife scheme makes it mandatory for newly qualified midwives to spend a year working in rural communities. And in Pakistan, a public sector programme for training and deploying female health workers has created a workforce of more than 90,000 covering 70% of the rural population.

So with less than four years left to meet Millennium Development Goal 4, the event on the 20 September is vital. Governments, the private sector, international institutions and civil society organisations will have the chance to review what has been achieved - and what more needs to be done - about the UN Secretary General's Every Woman, Every Child strategy, launched a year ago. That focused on the challenge of recruiting, training and supporting health workers.

On that day we need firm commitments. We need to see action at a global and national level: increased long term investment to recruit and train health workers, enough money to pay them a fair wage and give them support, training and equipment. Developing countries must put together plans for a better workforce - and be given aid to be able to put these plans into action.

At present one billion people - an eighth of the world's population - will never see a health worker at any point in their lives. By committing to real action on the 20th September, there will be the chance to save 15 million children's lives. What could be a more fitting birthday present for our seven billionth child?

Mark Malloch Brown is a member of Save the Children International Board, a former Minister of State in the Foreign and Commonwealth Office of the British government with responsibility for Africa, Asia and the United Nations. He is also a former Deputy Secretary General of the UN.