Celebrities such as Victoria Beckham often hit the headlines for quick weight loss after pregnancy
Pregnant women do not need to "eat for two", drink full fat milk or even alter how much food they eat for the first six months, NHS experts say.
In the last three months they only need an extra 200 calories a day, draft advice from the National Institute for Health and Clinical Excellence says.
It also urges women to have a "realistic expectation" of how long it will take to lose weight after birth.
The myth-busting guidance is now out for consultation.
Women trying to get pregnant who are obese - body mass index over 30 - should be advised about the increased risk to themselves and their babies, the guidelines, which are aimed at GPs, health visitors, midwives, and other health professionals, state.
Encouragement and advice on losing weight before pregnancy should be offered for this group, it says.
Pregnant women, especially those who are overweight or obese, should be encouraged to eat a healthy diet and do exercise.
But weight loss during pregnancy should not be advocated.
Women need to be aware that a moderate amount of exercise will not harm their baby and women who did exercise, such as running or aerobics, before pregnancy should be able to continue with no adverse effects.
The guidelines, which are out for consultation until 18 March, also cover losing weight after giving birth.
Women should be sensitively encouraged to lose excess weight but be warned that sensible gradual weight loss will take time - in contrast to some high profile celebrities who seem to reach pre-baby weight within weeks of childbirth.
The recommendations also point out that eating healthily and taking regular exercise will not affect the quality or quantity of breast milk.
Professor Mike Kelly, director of the centre for public health excellence at NICE, said: "In today's society 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.
"We want all women to be supported before, during and after they have children so that both they and their babies have the healthiest outcome possible."
He added that advice on healthy eating and physical activity for women after they have had a baby should take into account the demands of looking after a small baby and how tired the women are and any health problems they may have.
Jane Munro, from the Royal College of Midwives, welcomed the guidance and said weight was often an issue of concern for most women.
"It will be particularly helpful in dispelling post-pregnancy myths.
"Women need to understand that losing weight after giving birth can be a slow process and that physical exercise and gradual weight loss will not impact on their ability to breastfeed."
Mr Tahir Mahmood, vice president of the Royal College of Obstetricians and Gynaecologists said, the incidence of maternal obesity was increasing in the general population.
"We need to look at new evidence on the effects of weight gain on mother and baby, and the lifestyle and medical interventions that can help make lives healthier.
"Equally, we need to get the message across that mothers should be encouraged to work towards reaching their pre-pregnancy weight as this is an important indicator of future health."