What happens before birth shapes how your body responds to the external environment. Take dieting, if you go all out to keep it low calorie during pregnancy, your baby will think it is being born into a famine and its metabolism will be set accordingly. If mum's do not have adequate nutrition, the baby will be programmed to conserve energy and nutrients. Naturally when born their mothers make sure they are well fed and so begins the cycle once again. They store nutrients and become overweight adults and begin a life of dieting and probably pass the same behavior on to future generations.
It is not only important to eat good food, its important to eat enough food. Eat well and nourish both the baby, and yourself.