Yes, regular weight training will result in increased muscle mass. However, there is a certain type of training necessary to achieve these gains in size, training that involves heavy resistance and volume. By keeping the weight at about 40 to 50% of maximum effort and increasing repetitions for a cardiovascular effect, you will notice tone, not bulk.
You should eat a healthful, well-balanced diet during pregnancy. However, you should avoid certain foods, including raw or undercooked fish, poultry and meat; raw or partially cooked eggs or foods containing raw eggs; unpasteurized juices; raw sprouts; unpasteurized milk products; and some soft cheeses (cream cheese is OK). Avoid deli meats and frankfurters unless they have been reheated to steaming hot before eating. To prevent food-borne illnesses, take the following precautions:
If you lose weight suddenly or for unknown reasons, talk to your health care professional immediately. Unexplained weight loss may indicate a serious health condition. And even if it doesn't, simply being underweight is linked to menstrual irregularity, menstrual cessation (and sometimes, as a result, dental problems, such as erosion of the enamel and osteoporosis) and a higher risk of early death.
In the past, women have often tried to make up deficits in their diet though the use of vitamins and supplements. However, while supplements can be a useful safeguard against occasional nutrient shortfalls, they can’t compensate for an unbalanced or unhealthy diet. To ensure you get all the nutrients you need from the food you eat, try to aim for a diet rich in fruit, vegetables, quality protein, healthy fats, and low in processed, fried, and sugary foods.