Potassium maintains fluid and electrolyte balance in your body’s cells and helps your muscles contract. It is an electrolyte that will also help prevent leg cramps while pregnant. Since your blood volume expands by up to 50 percent during pregnancy, you’ll need slightly more electrolytes to keep the extra fluid in the right chemical balance. Potassium also helps to control blood pressure. Best Sources: Beet greens, swiss chard, sweet potatoes, potatoes, spinach, avocado.