Most doctors will tell you not to diet while pregnant. But it is not a bad time to control what you are eating. However, I’ve known people who have lost weight during their pregnancies because of better eating habits. Some sites have a pregnancy meal plan so that you get the right nutrition and don’t gain unnecessary weight. Following a good meal plan, you will be able to gain only the pregnancy weight (baby, extra fluid, placenta, breast tissue increase) that usually is lost right away after birth.
The weight gain during pregnancy is a normal and healthy occurrence, providing the nutrients the baby needs in order to grow. Even if you are overweight and become pregnant, you will still need to gain some weight; your body and developing child have specific nutritional needs that requires to be met. If you diet, you risk developing nutritional deficiencies to your baby.
According to one mom I know:
I ate healthy foods as possible! Minimal carbohydrates (one serving aka 15grams and had maybe two per meal and one at snacks) lots of fresh foods only, if I wanted a big greasy burger I made it myself instead of burger king and such,