Human reproduction is the result of the union of a spermatozoon with an egg. Eggs are produced in the ovaries and sperm in the testes. This union will result in an embryo that will develop in the uterus, resulting in the birth of a child.
If there is intercourse during the fertile days, the sperm will be deposited in the vaginal cavity. The sperm will then go through the cervical canal towards the uterine cavity and through the proximal tubal ostium to reach the fallopian tubes.
The fallopian tube will then receive the egg that was recently released with ovulation, via its fimbria. Sperm and egg meet in the ampulla of the fallopian tube and this is where the egg will be fertilised by only one sperm. The fertilised egg (zygote) will remain in the fallopian tube for 5-6 days, where it divides into 2, 4, 8 cells and then becomes a morula and a blastocyst.
As a blastocyst (an embryo with around 6-120 cells) it will then migrate towards the uterine cavity and implant by invading the endometrium where it continues to develop.
After conception
If there will be conception, the embryo begins to create its placenta at the point of implantation. The placenta secretes hormones, which stabilize the endometrium and prevent its degeneration. Menstruation does not come and the embryo continues its development normally, which the woman perceives with the delay of menstruation. Conversely, if one of the previous stages does not work (and at the right time), there is no pregnancy: the luteal phase of the cycle ends with endometrial cell degeneration (apoptosis) about 14 days after ovulation. The endometrium degenerates and falls with blood. The blood is drained through the cervix to the outside of the body, so the woman realize it as menstruation (period).
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