Detailed explanation of your baby\’s development at four months of pregnancy. At four months of pregnancy, your baby has already gone through an important stage of development. During this period, your baby is growing very quickly and many of his major organs are starting to form and start functioning. Here\’s how your baby develops at four months. Growth and Development When you are four months pregnant, your baby\’s body is already taking shape. At this point, your baby is about 5 inches (12 centimeters) long and weighs about 4 ounces (113 grams). The baby\’s length will increase at an alarming rate at this time.