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.