The size of a three-month-old baby includes a three-month-old baby in intrauterine pregnancy and a three-month-old baby at birth, and the sizes of three-month-old babies in the two different cases are. 1. intrauterine pregnancy three months baby: three months baby body length 9 cm, top hip length 6~7 cm, ultrasound can see the fetus in the uterine cavity complete human form, fetal limbs in the uterine cavity activities. At this time, the external genitalia of the fetus has developed, and the differentiation of fingers and toes as well as the development of skin and nails and liver appear. 2. Three months after birth: There are obvious individual differences in the size of babies in the first three months of life, which are related to the baby’s gender, nutritional status, feeding, eating and other factors. Usually the weight increases rapidly in the first three months, with an average increase of about 800 to 1200g per month. Both third trimester and birth trimester babies should be checked regularly at the hospital.