Answer:
first
Explanation:
The placenta develops from the trophoblasts, the blastocyst, which is connected to the umbilical cord and ensures the supply of nutrients and oxygen to the child, as well as the elimination of its metabolic products.
In the first month of pregnancy, the premature embryo must make a tremendous effort. During the third week it migrates from the fallopian tube to the uterus and nests in the endometrium at the beginning of the fourth week.
This process is also known as nesting. Shortly before, the embryo leaves the membrane that surrounds the ovule and penetrates so deeply into the mucous membrane that it is completely covered by it. If the nesting is successful, leave behind one of the riskiest stages of the first weeks of pregnancy: pregnancy is very likely to continue.
