The placenta begins forming soon after implantation, but it becomes fully functional by the end of the first trimester (around 12 weeks gestation). At this point, the placenta effectively takes over the roles of nutrient and gas exchange, hormone production, and waste elimination from the embryo. Prior to this, the embryo relies on the yolk sac and uterine secretions for nourishment. Full placental function is critical for sustaining fetal growth and development throughout pregnancy. This transition marks a key physiological milestone in gestation.

Reference: Williams Obstetrics, Cunningham et al., 25th Edition.