Question

why placenta is formed just after fertilisation explain and also writes its function

Answer 1

The placenta has several functions in the development of the human embryo. It is an endocrine organ, i.e an organ which secretes hormones from endocrine glands which are ductless. It is involved in the exchange of substances between the maternal and fetal circulation, e.g. glucose, water, inorganic ions, antibodies, fatty acids, amino acids, etc.
therefore, to provide the growing embryo all this and to carry out these functions for a healthy baby placenta is formed just after fertilization