All of your body's systems need water. When you are pregnant, your body needs even more water to stay hydrated and support the life inside you. Water also helps prevent constipation, hemorrhoids, excessive swelling, and urinary tract or bladder infections. Not getting enough water can lead to premature or early labor.
Your body gets the water it needs through the fluids you drink and the foods you eat. How much fluid you need to drink each day depends on many factors, such as your activity level, the weather, and your size. Your body needs more fluids when it is hot and when you are physically active. It also needs more water if you have a fever or if you are vomiting or have diarrhea.
The Institute of Medicine recommends that pregnant women drink about 10 cups of fluids daily. Water, juices, coffee, tea, and soft drinks all count toward your fluid needs. But keep in mind that some beverages are high in sugar and "empty" calories. A good way to tell if your fluid intake is okay is if your urine is pale yellow or colorless and you rarely feel thirsty. Thirst is a sign that your body is on its way to dehydration. Don't wait until you feel thirsty to drink.
This answer is based on source information from the National Women's Health Information Center.