The embryo, similar to a "ball" of cells, reaches the uterus. There it is implanted, that is, it attaches to the uterine mucosa approximately eight days after fertilization. This fixation in the uterine mucosa is called nesting.
The small embryo, formed from the zygote, may develop in the womb, protected by membranes and amniotic fluid. As early as the first weeks of pregnancy, the placenta forms.
The Importance Of The Placenta
The placenta is formed by tissues of the embryo and the maternal uterus and is typical of mammalian organism. The placenta attaches to the embryo through the umbilical cord, which has vessels through which blood circulates with oxygen and nutrients (which go from mother to fetus), and carbon dioxide and unused nutrient remains (these will from the fetus to the mother).
A pregnant woman who smokes or uses alcohol or other drugs, including certain medicines, may have a small placenta, which may compromise fetal development. Throughout pregnancy, the fetus grows and is protected within the womb. The navel marks the place where the child was connected to its mother through the umbilical cord.
In the first twelve weeks most organs are formed, including the heart, lungs, and kidneys. In the rest of the gestation period, the fetus grows and strengthens, making it fit for life in the environment outside the womb. It usually takes nine months (about 40 weeks) before the baby is ready to be born.