The first trimester is a time of rapid development. After fertilization, the embryo begins to form vital structures like the brain, spinal cord, and heart. By week 6, the baby’s heart starts beating, and tiny limb buds appear. By week 10, all major organs are in place, and the embryo transitions into a fetus. Many moms-to-be experience morning sickness, fatigue, and hormonal changes during this stage.