The second trimester begins at 14 weeks and ends at 26 weeks. During this time, the fetus’s immune and respiratory systems develop; hair begins covering its body; and it becomes capable of responding to sounds.  The third trimester begins at 26 weeks and ends with labor.  During the third trimester, the fetus’s lungs begin simulating breathing motions; its eyes start sensing light; its fingertips fully mature; and it begins its descent into the pelvis. Watch this video to learn all of the changes a fetus goes through!