Key Milestones in Fetal Development 💞 . . Fetal development is a complex process that occurs in several key stages throughout pregnancy. Here are some important milestones: First Trimester (Weeks 1-12) Weeks 1-2: Fertilization occurs, and the zygote begins to divide and develop. Week 3: The embryo implants in the uterine wall. Basic structures begin to form. Week 4: The heart starts to beat, and the neural tube (which will become the spine and brain) closes. Week 5: Limb buds appear, and facial