Fluid accumulation within the amnionic cavity, that is, between embryo and chorionic sac leads to a complete separation of embryo and surrounding extraembryonic tissues, only leaving the developing umbilical cord as the connection between placenta and embryo. The amnionic mesenchyme comes into direct contact with the chori-onic mesoderm lining the inner surface of the chorionic sac. Both tissue layers do not fuse, and it remains that amnion and chorion can easily slide against each other. As described above, it is only at the implantation pole that the definitive placenta develops. Due to regression of villi, most of the surface of the chorionic sac (about 70%) develops in such a way that the early chorionic plate, together with the amnion, remnants of villi and the early basal plate fuse and form a multilayered compact structure termed the chorion laeve or fetal membranes.
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The first trimester is very important for the mother and the baby. For most women it is common to find out about their pregnancy after they have missed their menstrual cycle. Since, not all women note their menstrual cycle and dates of intercourse, it may cause slight confusion about the exact date of conception. That is why most women find out that they are pregnant only after one month of pregnancy.