You won't be aware of your developing baby's activities inside the uterus for some months yet, but the fact that his elbows are forming allows him to make some small movements; the wrists do not yet move.

Your baby is looking more human by the day. His vertebrae and ribs are now in place, and his fingers are gradually lengthening. His body is less curled up than it was a few weeks ago.

The skeleton will gradually calcify and harden. With the exception of the cranial skull bones, all your baby's bones have a soft cartilage core that will later be reabsorbed as it is converted into hard bone. This process of hardening, known as ossification, starts in so-called primary ossification centers during the next five weeks of your pregnancy. Within these primary centers, specialized cells form spongy then hard bone as calcium salts are laid down. Within the hard bone is red bone marrow, which in later weeks will become the main producer of the baby's red blood cells.

Secondary ossification centers develop in the second trimester at the ends of each of your baby's bones.

The portion of bone between the hardening primary and secondary centers is known as the growth plate. This plate is responsible for the continued lengthening of your unborn baby's bones.