Live-born infants born before 37 full weeks of gestational age are called preterm or immature infants. Most of them have a birth weight of less than 2500g and a head circumference of less than 33cm. If their organ function and adaptability are poorer than those of full-term infants, they should still be given special care. Where the birth weight is reduced to below the 10th percentile of normal weight for that gestational age or below two standard deviations from the mean due to factors such as placental insufficiency, the baby is called less than gestational age (small sample, poorly mature child). Also referred to as low birth weight infants with birth weight below 2500g, and those with birth weight below 1500g are called very low weight infants, which include preterm infants and those younger than gestational age. The more premature the baby is, the thinner and more tender the skin, the more watery the tissue, the indentation, the red color, the less subcutaneous fat, the less muscle, the shorter and softer the nails, the longer the fetal hair on the trunk, the less and shorter the head hair, the larger the head, the wider the fontanelle, the flat and soft ear shell against the skull, the soft thorax, the areola is dotted, the edge does not protrude, the mammary glands are small or cannot be felt. The abdomen is more distended and the scrotum is poorly developed. In male preterm infants, the testes are often in the external groin and gradually descend into the scrotum during development. In females, the more premature the baby is, the more the labia minora are separated and prominent. There are few wrinkles on the soles of the hands and feet.