Q: 

1. Why cannot blood, which bleeds during periods, be extracted prematurely and used for good?
2. How does the fetus breathe through the mothers blood, especially when blood group is different, how come the blood does not get mixed up? How does oxygen get transferred?

A:

1. The human menstrual cycle involves a complex and regular change in female anatomy and physiology over an approximate monthly time period. This cycle commences at puberty and ceases at menopause. It prepares the lining of the uterus, under the influence of hormones, to receive a fertilised egg; failing which the hormone levels decline and the lining is shed. A small amount of bleeding accompanies this loss of endometrial lining of the uterus. It is not clear from your question what you mean by extracting blood prematurely? The blood loss is a normal physiological response of the body. It can be prevented by the use of hormones (the contraceptive pills) but there is no way to extract blood.
2. The precursor cells of the human placenta and the trophoblasts first appear four days after fertilization and they differentiate into all the other cell types found in the human placenta. When fully developed, the placenta serves as the interface between the mother and the developing fetus. The placenta is critical for a successful pregnancy by mediating such critical steps as implantation, pregnancy hormone production, immune protection of the fetus, increase in maternal vascular blood flow into it, and delivery. Over the next few weeks the placenta begins to make hormones which control the basic physiology of the mother in such a way that the fetus is supplied with the necessary nutrients and oxygen needed for successful growth. The placenta also protects the fetus from immune attack by the mother, removes waste products from the fetus, induces the mother to bring more blood to the placenta, and near the time of delivery, produces hormones that matures the fetal organs in preparation for life outside of the uterus. The placenta is the fetus extension into the mother, where it functions as the interface between the two. Like the radiator of a car, which is a heat exchanger, the placenta is a nutrient and waste exchanger. The fetal circulation enters the placenta much like the water of an automobile engine enters the radiator via the umbilical arteries embedded within the umbilical cord. Once in the placenta, the fetal circulation branches into units called cotyledons, structures similar to inverted trees. The finest branches of the fetal circulation are made up of capillary loops within the chorionic villi. Once nutrients have been absorbed and waste products released, the fetal blood ultimately collects into the umbilical vein, where it returns to the fetus via the umbilical cord. If the fetal circulation is similar to the circulating water in an engine, the maternal circulation is analogous to the cool air rushing by the fine fins of the radiator. The maternal blood enters the placenta via the spiral arteries of the uterus. At the point were the spiral arteries make contact with the placenta, they end in open channels, fountaining maternal blood into the intervillous space. The intervillous blood is returned to the maternal circulation via drain-like uterine veins. In order to support the developing fetus, especially at term, up to 35% of the maternal blood flow courses through the intervillous space.