After losing massive amounts of fluid during childbirth, then producing large amounts of breast milk, your postpartum body requires all the water it can possibly get. In the first few weeks after having my daughter, I was constantly thirsty and had to drink a gallon and a half per day to meet my body's needs. This not only prevents dehydration, but can also significantly help with postpartum weight loss. Many studies have shown that overweight women benefit from drinking more water; it suppresses the appetite and promotes fat-burning.
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