During the fourth week of pregnancy, the blastocyst-state embryo implants in the endometrium, that is, it adheres to the inner layer of the uterus and invades it.
From the trophectoderm of the embryo, will form the placenta, the organ responsible for nutrition of the baby throughout pregnancy, and the umbilical cord. The cells of the syncytiotrophoblast proliferate towards the endometrium breaking the blood vessels and the cells of the citotrophoblast form the primary chorionic villi, thereby establishing the uteroplacental circulation.
On the other hand, the amniotic cavity is formed, a place filled with amniotic fluid where the fetus will develop in suspension. The amnion is the membrane that limits this sac that contains the baby.
Finally, the entire network of blood vessels between the mother, fetus, and syncytiotrophoblast forms a structure called the chorion.
