Mothers who eat too much during pregnancy increase the risk of low IQ, eating disorders and psychosis in their children.

Every pregnant woman worries about eating properly to nurture her unborn child.

But between strange cravings and old-fashioned advice to start "eating for two" many find themselves tipping the scales far too heavily - with one in six women classed as obese within just three months of becoming pregnant, and many more putting on too much weight in later months - suggesting babies are being put at risk on a massive scale.

Now, a study has warned of new risks to their children, who are more likely than others to suffer from a range of problems including low IQs, eating disorders and psychosis.

The research found that the risks were increased not just in the early years - when children had a higher chance of suffering from attention deficit disorder, and a lower than average IQ - but also in adolescence, when eating disorders were more likely, and in adulthood, when the risk of disorders such as schizophrenia was increased.

Experts warned that even women who began their pregnancies at healthy weight, or slightly overweight, could put their child at risk if they used pregnancy as a time to ditch sensible eating habits.

The research, published in international journal Obesity Reviews, examined findings from across across the world on the impact of maternal weight on child development.

One study found that every increased unit in the pregnant woman's Body Mass Index (BMI) - calculated as her weight in kilograms divided by the square of her height in metres - was associated with a "significantly" reduced IQ in the child.

Overall, the average IQ of children of obese mothers was five points lower than those born to mothers of healthy weight, according to the study, one of 12 examined by researchers in psychiatry and behavioural neuroscience from McMaster University, in Ontario Canada.

Research from Sweden found children born of overweight mothers were more likely to suffer from attention deficit problems, while findings from Japan found every extra BMI point added in early pregnancy increased the offspring's chance of developing schizophrenia in adulthood by 24 per cent.

A study of Australian teenagers suggested their chance of having an eating disorder increased by 11 per cent for every extra BMI point gained by their mothers during the pregnancy.

Researchers said the reasons for the extra risks were not clear, but might be linked to changes in the hormonal, cardiovascular and immune systems during pregnancy as a result of excess weight.

While the studies attempted to "control" for some factors - such as the socio-economic background of obese parents, which could affect child development - the research could not pinpoint the extent to which risks were created in pregnancy, or might explained by genetic patterns or the way parents brought up their families.

Research has found that one in six women in this country are obese by the time they are three months into pregnancy - a proportion which has more than doubled in two decades, amid an obesity epidemic.

Earlier this year, scientists said more than 100,000 babies a year were at risk of dying or suffering serious problems because of the risks from obesity, which puts a woman's body under extra strain, when it is working to nurture new life.

Obesity in pregnancy increases the risk of diabetes, high blood pressure, blood clots, miscarriages and still-births, while layers of fat can mean serious defects can be missed in scans.

The study, led by Teeside University, found massive variations in levels of obesity among expectant mothers cross the country.

In London, 13 per cent of mothers-to-be were obese, while in the West Midlands, the figure was 22 per cent.

Tam Fry from the National Obesity Forum said too many women were not getting the message that they needed to "shape up" before planning a family.

Others who were a healthy weight, or slightly overweight when they conceived could easily fall into the category of obese if they used pregnancy as an excuse to ditch sensible eating habits.

He said: "There is still a lot of folklore about pregnancy, and a lot of women who still believe that 'eating for two' is the way to nurture their child.

"Those kinds of myths are really dangerous - in fact, pregnant women only need about an extra 200 calories a day in the last trimester."

"There are a lot of serious risks from obesity that we already know about, but this study shows others are still emerging, and the picture is incredibly worrying," he added.

Earlier this year, a study found that rates of foetal and infant death are twice as high among those born to obese mothers.

The study found that 16 in 1,000 babies suffered fetal or infant death if their mothers had a BMI greater than 30 in early pregnancy, compared with 9 deaths per 1,000 babies among those with a healthy BMI.

Big babies - those weighing more than 10 lbs - are around twice as likely as other babies to end up obese as adults.

Separate studies have found increased dangers of developmental problems like spina bifida and neural tube defects in infants of obese mothers.

Jane Munro, from the Royal College of Midwives, said more research was needed to establish whether the apparent risks highlighted in the latest study were a direct result of maternal obesity.

Mrs Munro said: "There are some clear risks from obesity that we already know about- hypertension, pre-eclampsia, larger babies, an increased risk of having a baby born by Caesarean section, and that the child is more likely to become obese.

"We encourage women to get to a healthy weight before conception, and eat healthily in pregnancy, but we do not encourage dieting in pregnancy, and we don't want women getting too frightened about all this."