Menstrual cycle followed by pregnancy

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If the oocyte is fertilized after ovulation, it continues to progress in the fallopian tube while the journey from oocyte to embryo ( 2 cells embryo then 4, 8 cells and blastocyst ... ) is going on. Arriving in the uterus it implants there by sinking into the carpet lining of the endometrium. The endometrium itself is a transformation of the outer layer of the uterus, which has evolved during the cycle in order to be able to receive embryo.
The formation of the chorion begins under secretion of a specific hormone, the human chorionic gonadotrophin, or HCG. That is this homone which is used for the immunological diagnosis of pregnancy. Several types of urine and blood tests are disposable to reveal the beginning of a pregnancy. Blood quantitative tests are more sensitive and allow an earlier diagnosis, but most of the tests which can be purchased in pharmacies are perfectly reliable if used a few days (about a week) after the end of theorical menses. These tests are performed on morning urines. Their reliability depends on full respect of the procedure (i.e. delay required for results).