Growing and maturing. While the organ systems and body first form in the beginning of pregnancy, they continue to mature and grow until birth. Muscles are getting stronger, the brain continues to develop, and the baby is practicing breating and swallowing. One good site to track development is american baby magazine website. Lennart nilsson's book "a child is born" is an excellent reference you can buy.
In the 3rd.... ..Trimester, development of organs is basically finished, with a few exceptions; growth/weight gain and completion of development of lungs, brain, and blood vessels are all 3rd trimester events.