During your first trimester of pregnancy, your body mostly undergoes invisible, chemical changes. By the second and third trimester, your growing belly causes aches and pains, but your first 12 weeks can pose challenges because of hormonal shifts, nutritional needs and low blood pressure. Talk to your prenatal caregiver about the best ways for you to handle the new demands pregnancy places on your body.NutritionAs soon as you become pregnant, your body begins diverting nutrients from your diet to your baby. This means that you need more calories, vitamins and minerals, especially the ones that help your baby develop. Increase the amount of protein you eat, and keep your diet as healthful as possible. Eat mostly whole grains, lean protein, fruits, vegetables and low-fat dairy products. Talk to your doctor about how to get enough iron, calcium, vitamin D and folate -- the nutrients particularly important for pregnant women -- and how many calories you need per day.ExerciseExercising during your first trimester can relieve stress, help you stay strong and improve your sleep. According to the American Academy of Family Physicians, most pregnant women can continue their regular exercise program. If you don't normally exercise, begin walking, swimming, practicing yoga or doing Pilates for 30 minutes each day. Changes in your blood pressure and blood sugar during your first trimester may cause dizziness or fatigue while exercising. If you experience these or other discomforts, stop exercising or slow down until you feel better. Talk to your doctor before beginning an exercise program.Common AilmentsAs your body redirects nutrients to your baby, you may become fatigued more easily than usual, particularly during your first trimester. Get plenty of sleep each night, and nap during the day if you need to. Fatigue may be a sign of anemia, so talk to your doctor if you are often tired. If you experience morning sickness, try eating a handful of crackers as soon as you wake up, and eat small, mildly flavored meals throughout the day. Although nausea is unpleasant, it is common during the first trimester and is not usually a cause for concern.WarningsDo not smoke, drink alcohol or take drugs while you are pregnant. Limit your coffee intake to 1 or 2 cups per day, and do not drink unpasteurized milk or juice. Avoid soft cheeses, undercooked meat or eggs and fish that may contain mercury. Ask your doctor for a full list of foods to limit or eliminate during pregnancy, and talk to him about any medications you take. If you have a cat, recruit someone else to change the litter box because it may carry toxoplasmosis, a disease that can affect your baby.When to See a DoctorMild dizziness, morning sickness, headaches and constipation are common during the first 12 weeks of pregnancy. However, if any of these symptoms become severe or persistent, see your doctor. If you experience changes in vision or abdominal pain, develop a fever or infection, or have any other concerns, see your doctor. Schedule regular prenatal appointments every four weeks during the first trimester so your health care provider can track your pregnancy and catch any possible complications early.