Mothers-to-be should be warned that 'eating for two' is a myth, according to draft Health Service guidelines.
They do not need to drink full-fat milk or change their diet at all for the first six months of the pregnancy.
Even in the last three months they need just 200 extra calories a day - the equivalent of a small sandwich.
New advice on weight management during pregnancy comes as the number of obese mothers is rising, with almost one in four women being obese and a further third overweight.
It says women should be advised that being fat puts their baby at risk, but not told to lose weight.
Instead they should be helped to shed excess pounds before getting pregnant and after they have given birth.
The guidance from the National Institute for Health and Clinical Excellence (NICE) is now out for consultation.
Professor Mike Kelly, Director of the Centre for Public Health Excellence at NICE, said: 'Women are bombarded by often conflicting advice on what constitutes a healthy diet and how much physical activity they should do during pregnancy and after birth.
'The aim of developing this new guidance is to provide health professionals with clear recommendations to help them support women prior to and during their pregnancy as well as after they have given birth.
'Many overweight women have healthy babies, but the evidence suggests that there are more risks associated with pregnancies in women who have a BMI of over 30.'
He said the advice takes into account the demands of looking after a small baby and how tired mothers are.
'But it also aims to dispel any myths about what and how much to eat during pregnancy - there is no need to "eat for two" or to drink full-fat milk,' he said.
'It's important for women to understand that weight loss after birth takes time and that physical activity and gradual weight loss will not affect a woman's ability to breastfeed.'
The Royal College of Obstetricians and Gynaecologists says the recommended maximum limit for pregnancy weight gain is 10-12 kilograms, around one and three quarter stones.
Rosie Dodds of the National Childbirth Trust said: 'Women are more likely to make changes to their diet when they are pregnant and this opportunity can improve the family eating pattern for the future.
'NCT welcomes this draft guidance which should ensure better consistency of support from health professionals and tailoring of the services offered to the needs of women.
'In some areas, women on low incomes need improved access to affordable nourishing foods, especially fresh fruit and vegetables
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