The menstrual cycle
In each natural cycle a cohort of 8-12 follicles start developing under the effect of FSH. One of these becomes the leading follicle, grows faster than the rest of the cohort and is called the dominant follicle. This is the follicle that will reach the final maturation. The rest of the follicles will become atretic, i.e. they will degenerate via atresia. Under the action of LH the dominant follicle will reach ovulation and a mature egg will be released.
While the follicle develops it produces estradiol (E2) which acts on the lining of the womb (endometrium) increasing its thickness. High levels of estradiol lead to the secretion of LH. The sudden rise of LH (called LH surge) is the biological signal for the final maturation of the egg and its release from the follicle (ovulation). This stage of the cycle corresponds to the fertile days. After ovulation the follicle is transformed into a corpus luteum, which mainly produces progesterone. Progesterone combined with estrogens (estradiol) prepare the endometrium to receive and support the embryo.
IVF mimics the natural mechanisms of conception.