During the first trimester, which is from weeks 1 to 12, the baby starts to develop major organs, such as the brain, heart, and spinal cord. Throughout these weeks, the baby’s head, face, and ears, as well as its arms and legs, will start to develop. Around week 8, your baby is slightly larger than a pencil eraser. Around week 11 is when your baby is officially considered a fetus. By the end of the first trimester, your baby will barely measure 2 inches.