Pregnancy Week 4 — Implantation Complete & Early Embryo Development. Week 4 is an important milestone in early pregnancy. The embryo is now securely implanted in the uterus, pregnancy hormones are increasing, and the placenta begins forming to support the baby’s growth. Although the baby is still extremely tiny, rapid cell division is happening and the earliest structures that will become organs are starting to develop. Many women also begin noticing early signs like fatigue, breast tenderness,