It is not just the thickness and exact histological nature of the placental barrier that defines the rate of diffusional exchange. Another important determinant is the direction of the blood flows of mother and fetus in relation to each other. The vascular arrangement of the human placenta cannot be clearly defined due to the branching of the vil-lous trees into all directions and a respective maternal blood flow somehow bypassing these branches. Therefore, this unpredictable and variable flow pattern has been termed multivillous flow (Fig. 3.1d).
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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.