Read the following statements about the pregnancy of humans:

I) Most of the major organ systems are formed by the end of first trimester.

II) The end of second trimester is the beginning of the foetal heart sound.

III) The first movements of the foetus usually observed in the middle of second trimester.

IV) The placenta is formed around the developing foetus in the beginning of third trimester.

Which of the statements given above are correct?