Fetal body development Four months into pregnancy, the fetus\’s body begins to develop rapidly. It is about 13.5 centimeters long and weighs about 140 grams. At this point, your baby\’s head is fully developed and takes up proportion of the body, but over time other parts of the body will continue to grow, especially the hands, feet, and organ systems. At four months, the fetal heart is fully developed and can distinguish a stethoscope like an adult.