Which organ undergoes development and becomes fully functional during the first trimester of pregnancy?

The organ that undergoes significant development and becomes fully functional during the first trimester of pregnancy is the placenta. The placenta forms shortly after fertilization and continues to develop throughout the first trimester. By the end of this trimester, it is fully functional, playing a crucial role in providing oxygen and nutrients to the developing fetus while also removing waste products. The placenta connects to the fetus through the umbilical cord and is essential for maintaining a healthy pregnancy. Additionally, it produces hormones that help regulate various processes in the mother's body, ensuring a supportive environment for the fetus’s growth and development.