Fetal Development (Week-by-Week Overview) Fetal development refers to the growth and maturation of the baby inside the uterus, starting from fertilization until birth. It is broadly divided into three trimesters, each with specific developmental milestones. First Trimester (Week 1–12) Foundation phase – organs are formed This is the most critical period, as all major organs begin to develop. → Week 1–2 Fertilization occurs → zygote forms → travels through fallopian tube → implants in the uterus.