The birth weight of infants is related to the gestational age, gender and intrauterine nutritional status. The average birth weight of male infants in China is 2.98-3.78 Kg, and that of female infants is 2.86-3.66 kg. Within one week after birth, due to insufficient milk intake, water loss and meconium excretion, a temporary weight loss, or physiological weight loss, may occur. It reaches its lowest point on the 3rd-4th day after birth, and then gradually regains its weight to the 7th-10th day after birth.