If you saw a pregnant woman smoking a cigarette what would you do?

Would you rush to a cash point, withdraw £400 and offer it to her in instalments to persuade her to stop? What if I told you something similar was being done in your name, and with your taxes?

Would you think that was madness? Well, it is happening. In Tayside and Glasgow, £400 or thereabouts have been given to pregnant mothers as an incentive to quit smoking. And guess what: it works.

Women who might have blighted their baby's health, and their own, have overcome their nicotine addiction for the length of the pregnancy. Paying them is effective.

But does that make it right? Professor Linda Bauld of Stirling University, who ran the Glasgow research, is in no doubt. For her, paying pregnant mothers to stop smoking isn't a moral issue.

It's about seeing babies born healthy and witnessing young women have better pregnancies, thereby saving the NHS a fortune in the long term.

The evidence is hard to argue with. Ms Bauld took 600 pregnant women who said they wanted to stop smoking and divided them into two groups. One was offered advice and support, while the other was also offered a financial incentive.

The women received £50 when they signed up and (if blood, urine and saliva tests were clear) a further £50 four weeks later. After three months of not smoking, they were given £100. And if they managed to remain an ex- smoker all the way to their baby's delivery they collected a £200 lump sum.

All of the money was in the form of vouchers that were redeemable in supermarkets and high street shops. They didn't allow the purchase of alcohol or cigarettes but could be used to buy food and, according to Ms Bauld, were most often used for baby items such as cots and prams.

The key finding was that, while nine per cent managed to stop smoking all the way to the baby's birth without financial incentive, the figure rose to 21 per cent of those who were being paid staying off cigarettes.

In Tayside, where "Give it up for baby" has been running successfully since 2007, the figures are equally impressive. That scheme differs in that women are offered a steady £12.50 in vouchers from the moment they sign up until their baby is three months old.

Andrew Radley, consultant in Public Health Pharmacy, reported more than half of pregnant smokers signing up for the scheme. Thirty-three per cent were not smoking at the 12th week of pregnancy and 20 per cent were still nicotine free when they gave birth.

These figures make the schemes three times more successful than any other intervention. Andrew Radley sees the cost as money well spent. Ms Bauld agrees. She is putting in a proposal to a national funder for a multi-centre trial.

So should our hearts sink at the realisation that money must matter more to these mothers-to-be than the precious cargo that is their baby?

Well, first let's look at some other facts.

In affluent areas of Scotland, five per cent of pregnant women smoke. In areas of greatest deprivation almost 30 per cent do - nearly one in three. Most women give up as soon as they know they are pregnant. For many, cigarettes will join coffee and other favourite treats in tasting metallic or in making them feel nauseous.

Others will struggle but will manage to overcome their urge to smoke because the evidence is so clear that cigarettes can damage the baby. Others will carry on despite being told it might affect the baby's growth and brain development and increase the risk of still-birth.

The baby could also be lighter and be born prematurely with associated health problems. The list goes on.

It's hard not to feel condemnatory of women who are given this information and smoke anyway. Not surprisingly, it is a contentious subject of discussion on the Mumsnet website.

Some women try to justify their behaviour by citing examples of mothers having healthy babies despite smoking. But as one pithy contributor noted: "The plural of anecdote is not evidence."

And the evidence makes bleak reading. Smoking damages babies and costs the NHS vast sums every year.

But isn't paying mothers adding to the cost? The £750,000 spent on the Glasgow trial seems like a vast sum until it is set against the £6,000 average cost of admitting just one baby to intensive care.

That's the starting point for a potential life-time of costly ill health. If £400 is the cost of prevention, there is no financial argument worth having.

But what about the morality of bribing women to stop smoking when the danger to their baby is what should motivate them? Isn't it just plain wrong to reward them? Why should they get what other more virtuous mums could be doing with? Put in its simplest terms, why are they rewarded for being reckless?

Talking to the professionals, I had the impression this small sum of money might be the first break some of these women have had - ever.

Every mother knows that to be pregnant is a challenge as well as a joy. To be poor and pregnant is often just a challenge.

Many of these mothers have inadequate housing, difficult relationships, low self-esteem and only enough income to subsist. The vision of motherhood we see in advertisements and in catalogues is a far cry from what they experience. The prospect of a new baby may be overwhelming. How will they manage? They turn to a cigarette.

The experts say the money, albeit in the form of vouchers, often lifts the pressure. That little bit extra allows them to worry less and plan more.

They can buy better food or know they can gather up items the baby will need. The evidence shows that the money is the hand they need to pull them out of their addiction.

That's the benefit of approaching health problems from a practical perspective and of running trials to discover what works.

Is it right to reward the addicted for quitting? No. Shouldn't these women find the strength to do the right thing for their babies? Yes. But if we approach this only from a moral stand point what happens to their babies? Isn't rescuing them from damage the greater moral argument?

Surely we are obliged (society as well as mothers-to- be) to offer the next generation the best possible start in life.

Ideally we'd banish deprivation. Failing that, if £400 ensures a baby a nicotine-free passage through pregnancy then society should pay it.

It will allow him or her to arrive bigger and healthier. It will decrease the risk of asthma, eczema, lung problems and heart disease.

And that in turn will save society a multiple of what it has invested.

For some paying women to stop smoking will be a hard pill to swallow- but it's also a no-brainer.