The placenta begins to form shortly after a fertilized egg implants into the uterine wall, which happens around 7 to 10 days after conception. Initially, the placenta starts as a few cells and then grows throughout the pregnancy to become a substantial organ that supports the baby. At around week 4 of pregnancy, the placenta begins to develop further from the outer cells of the blastocyst, forming structures that establish a connection with the mother's blood supply. By about week 12, the placenta is fully formed and ready to take over nourishment for the baby, although it continues to grow and mature throughout the pregnancy. It is generally considered mature by around 34 weeks. The placenta contains many blood vessels and connects to the baby through the umbilical cord, facilitating the delivery of oxygen and nutrients and the removal of waste products. In summary:

• Placenta formation begins after implantation (7 to 10 days post-conception).
• It starts forming physically around week 4.
• It becomes fully functional by week 12.
• It continues to grow until around week 34 to full maturity.