The formation of cell columns does not result in a complete trophoblastic shell but rather in separated columns from which extravillous trophoblast cells invade maternal uterine tissues (Fig. 3.3). These cells migrate as interstitial trophoblast into the endometrial stroma, while a subset of the interstitial trophoblast further penetrates the wall of the uterine spiral arteries (intramural trophoblast), finally reaching the vessels' lumen (endovascular trophoblast) (Fig. 3.3). Some of the interstitial trophoblast cells fuse and generate the multinucleated trophoblast giant cells (Fig. 3.4) at the boundary between the endometrium and myometrium.
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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.