Pregnancy: how it all begins
Any pregnancy begins with the maturation of reproductive cells: in women, oocytes cyclically ripen, in men - continuously sperm. These processes are called ovogenesis and spermatogenesis, respectively. A specific feature of germ cells (gametes) is the presence in the nucleus of a half set of chromosomes, which leads to the fact that when two such cells merge, a zygote is formed with the usual set of chromosomes (46 chromosomes).
After the maturation of the germ cells, conditions are necessary for meeting the male and female gametes, that is, conception. Such conditions are directly sexual intercourse, the apogee of which is male ejaculation. After that, male gametes, equipped with special tails, make their way through the genital tract of the female to the ovum, which is waiting for sperm in one of the two fallopian tubes.
After the fusion of the egg and one sperm cell, the sex of the unborn child is determined: it depends on the presence of a chromosome in the sperm that is responsible for the sex. If a zygote with a set of chromosomes of the twentieth is formed at the merger, a girl will be born, and if XY - a boy.In this case, the eggs always carry only X chromosomes.
Next, the egg begins to divide (the process of crushing or the formation of multicellularity), a blastocyst is formed, which slowly descends through the fallopian fallopian tube into the uterus and is attached to the wall of the uterus on the seventh-tenth day (implantation). This period is called pre-payment. At the same time, the uterus is also preparing for the adoption of a fertilized egg: all the time that the gamete matures in the female body, a special lining grows on the walls of the uterus - the endometrium, which is an ideal substrate for providing the newly formed organism with everything necessary.
Pregnancy is conventionally divided into three stages - trimester. In the first trimester, which lasts 12 weeks (embryonic period), intensive formation and insertion of all future organs and body parts of the fetus (organogenesis) takes place, and a new organ is formed in the mother's body - the placenta (baby seat), which will nourish and protect the pregnancy , existing until the expulsion of the fetus from the womb (placentation).
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