It is the latest dilemma of modern parenting and it isn't getting any easier to solve. Here's the fix: you're pregnant, you want the very best for your baby's future, and then you read an advert that promises you a once-in-a-lifetime opportunity to safeguard your child's medical future. It invites you to "freeze a spare immune system" for your child, by banking the precious stem cells from the umbilical cord. If you don't bank them, they'll be discarded. If you do, they might - just might - save your child's life.

It doesn't sound too difficult a choice to make, until you consider that banking the stem cells will set you back somewhere in the region of £1,200, and that you may be being sold a false dream. In a paper in the journal PLoS Medicine this week, Nick Fisk and Irene Roberts, both professors at Imperial College London, warn that parents are being exploited at a time when they are at their most vulnerable. The possibility of your baby needing its own cord stem cells, they say, is less than one in 20,000 - and many of the applications that private stem-cell banks allude to in their publicity material, such as cures for heart disease or Parkinson's in the dim and distant future, are speculative and may never come to fruition.

The problem is that, though Fisk and Roberts are dubious, they never say never. "We say it's unlikely to be used, but we don't say it's like cryogenics - not like being frozen in the hope you'll one day be brought back to life," says Fisk. "There's a real chance it could be used, but it's a low chance." Low enough that his advice would be don't let it bother you if you decide it's not worth £1,200: but there again, if your surname happened to be Beckham and the price tag seemed like peanuts, it's probably not the nuttiest thing you could possibly do to have a bit of your new baby's cord blood preserved in a deep freeze somewhere - just in case.

Then again, in case what? Private cord banks usually give parents the impression that frozen stem cells could be used to treat the baby in whose umbilical cord they originated: but in fact, say doctors, it's more likely to be of use to siblings. There are various reasons for this: one is that, where diseases such as leukaemia are concerned, the seeds may be sown in the womb - so the child's own stem cells quite possibly carry the genetic defect that led to the condition, and so are useless in its treatment. What's more, there is some evidence that stem cells from a donor are more effective at fighting blood diseases than an individual's own cells.

There is, though, the possibility that medical knowledge will advance a lot more quickly than Fisk and Roberts anticipate, and that we could be just round the corner from stem cells being used to regenerate heart muscle and to make new and healthy nerve and muscle tissues when or if disease intervenes. People such as Dr Kees Kleinbloesem, director of the company Stems4Health, argues that the premise of Fisk's argument is dated - work is advancing all the time, and huge breakthroughs could happen. But even then, says Fisk, it's likely that medical research will at the same time be working on how to better harvest stem cells from within an individual's own body (an umbilical cord might be a good source of stem cells but it certainly isn't the only source - stem cells from bone marrow, for example, have long been used to treat leukaemia and other diseases).

And there is, too, the little-known work of the publicly funded national cord blood bank, which is quietly freezing the umbilical stem cells of around 1,200 UK babies every year in the hope that they can be used to treat people anywhere in the world who need tissue-typed stem cells. Mothers, usually in London hospitals, are asked if they'll consent to their baby's cord blood being banked: the cells are then tissue-typed and included on an international database. Currently about 2,000 people worldwide are treated each year from cord blood held in national banks, and doctors hope that as medical knowledge increases, more can be treated in the future. In a way, it's the opposite of what's happening in the private sector: there, the emphasis is on the uniqueness of the stem cells and their possible use for one child. With the public stem banks, the argument is about how stem cells, properly catalogued, could be a kind of insurance not just for one child, but for a whole generation - wherever they might be on the planet.