Obesity levels among pregnant women have reached epidemic levels, putting the health of their babies at risk, experts say.

The health watchdog NICE has issued new guidelines encouraging women in England to attain a healthy weight before they get pregnant.

It also advises them against eating for two once they conceive.

It says almost half of women of childbearing age are overweight or obese, which could harm their child.

Many women feel they are offered confusing and conflicting advice about their health during pregnancy.

The guidelines from the National Institute for Health and Clinical Excellence are aimed at cutting through that. They discuss weight and exercise before, during and after pregnancy.

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If a woman is obese during pregnancy, she has an increased risk of developing serious complications like pre-eclampsia, gestational diabetes, miscarriage and stillbirth. She is also more likely to have a Caesarean section.

It can set a worrying example, by encouraging new mums to embark on drastic weight lost programmes, which are not only unrealistic, but can also be very unhealthy Belinda Phipps, NCT

NICE says women with a body mass index of more than 30 should be encouraged to lose weight before they become pregnant. During pregnancy, losing weight can be harmful to the unborn child, so women are advised to eat healthily and to do gentle exercise.

After they have given birth, women are told they should lose their baby weight gradually. Experts from NICE say celebrities who regain their pre-baby figures very fast can put unrealistic pressure on ordinary mothers.

"Women should understand that weight loss after birth takes time, and physical activity and gradual weight loss will not affect their ability to breastfeed," said Professor Mike Kelly, NICE public health director.

"Losing weight gradually can actually help women maintain a healthy weight in the long term."

NICE also wants local authorities to offer women more opportunities to exercise, at an affordable price, and with creches for their children.

"There's been an exponential increase in obesity among pregnant women," said Professor Lucilla Poston, from King's College, London, who helped develop the guidance. "It's very worrying, as there are so many potential risks for the mother and her baby."

The National Childbirth Trust welcomed the move to bring clarity to the issue. Chief executive Belinda Phipps said women were presented with a lot of conflicting advice about exercise.

"It can set a worrying example, by encouraging new mums to embark on drastic weight lost programmes, which are not only unrealistic, but can also be very unhealthy," she said.

Professor Sir Sabaratnam Arulkumaran, president of the Royal College of Obstetricians and Gynaecologists, said he hoped the new guidelines would be actively promoted by the government.

"We need to support women who are overweight by encouraging them to lead healthier lifestyles. This includes providing them with advice on diet, nutrition and exercise.

"These healthy behaviours should occur throughout a woman's lifetime and not just when she is considering starting a family or during pregnancy."