Before birth, doctors use ultrasound to find a baby's gestational age. The doctor measures the size of your baby's head, belly, and upper leg bone. To check gestational age after your baby is born, the doctor will weigh and measure the baby and see how big your baby's head is. The doctor will also look at how well your baby has developed. The doctor will compare the sizes and how your baby has developed to charts showing how other babies have grown at the same age. These charts show what is normal growth and development for babies from about 25 to 42 weeks gestational age.