The chorion and the amnion together form the amniotic sac. The trophoblast layer differentiates into amnion and the chorion, which then comprise the fetal membranes. Initially, the amnion is separated from the chorion by chorionic fluid. The outer membrane, the chorion, contains the amnion and is part of the placenta. An amnion, chorion, and allantois are present during embryonic life. Then, as the twins develop, they will share the same placenta, chorion, and amnion. Thus the placenta develops from the chorion frondosum and the decidua basalis. The fusion of the amnion and chorion is completed at the 12th week of development. During pregnancy it is also produced by the placenta, chorion, and decidua. During childbirth, the detachment of the placental chorion can cause excessive bleeding. The placenta is diffuse and epitheliochorial, with a crescent-like chorion. MZDC twins are born in two different chorion sacs. This fuses with the chorion laevae, thus filling up the uterine cavity. The egg is covered by a hard-ridged protective outer layer of shell, called the chorion. Also, most marsupial allantoises do not fuse with the chorion.