Which organ develops and is fully functional in the first trimester?

The organ that develops and is fully functional in the first trimester of pregnancy is the heart. During the first trimester, specifically by the end of the embryonic period (around 8 weeks), the heart undergoes significant development. It starts as a simple tube and gradually transforms into a four-chambered organ that can pump blood effectively.

By the end of the first trimester (around 12 weeks), the heart has fully formed its chambers and the critical structures needed for circulation, which is essential for supplying nutrients and oxygen to the developing fetus. This early development of the heart is crucial as it supports the growth of other organs and systems in the body. Overall, the heart's early functionality is vital for the overall health and viability of the pregnancy.