What are those changes that women suffer in their bodies every 28 days? What are they for? What happens when fertilization occurs? What happens when it doesn’t?

Every month women bodies suffer a group of changes with the purpose of preparing itself to sustain a new life, a baby, this set of changes are what we know as menstrual cycle.

The menstrual cycle is divided in two phases separated by the ovulation, so in order we have:

Follicular phase. Ovulation. Luteal phase.

Follicular phase.

This phase begins the first day of the menstrual cycle and ends the day when ovulation happens.

A primordial follicle develops to a stage known as Graafian follicle and the follicles that couldn’t reach this stage will die.

It is important to know that during the first half of the follicular phase the levels of the FSH hormone will be higher than those of LH hormone and during the second half, levels of LH will be higher than those of FSH, have this in mind for the next paragraphs.

FSH will stimulate granulose cell in the primordial follicle to increase the estradiol synthesis, next the estradiol will produce an up-regulation of receptors for FSH and estradiol itself, this up-regulation will occur in a autocrine way, that is to say, the receptors for FSH and estradiol will increase on the granulose cells of the primordial follicle. The granulose cells will now produce growth factors that will increase tissue size and function, the number of granulose cells and theca cells, another type of cell inside the Graafian follicle, increases. The result is the formation of the Graafian follicle. Up to this point the estradiol creates a negative feedback that limits the secretion of FSH and LH.

Things change when the follicle reaches the Graafian stage, now the levels of estradiol will reach a threshold and the feedback, instead of being negative, will be a positive feedback, as a result the secretion of LH and FSH will increase and now the levels of LH will be higher than those of FSH. The abrupt release of LH produces instant changes inside the follicle that will end producing the ovulation, that is, the expulsion of the oocyte from the Graafian follicle. The estradiol levels will decrease after ovulation.

If there is no positive feedback ovulation doesn’t not occurs.

During the follicular phase the main ovarian production is estradiol.

Luteal phase.

Corresponds to the last 14 days of the 28 days menstrual cycle.

The LH will produce leutinization of the follicular cells that remain after the expulsion of the oocyte, the corpus luteum is formed as a result of this.

The corpus luteum will produce progesterone and this one will produce a negative feedback over the synthesis of LH and FSH, as a result the levels of these will gradually decrease during the luteal phase.

The progesterone increases the secretory activity and vascularization of the uterus, both needed for the development of a fertilized oocyte.

If fertilization doesn’t occur the corpus luteum will regress and the secretion of estradiol and progesterone will begin decreasing 8 days after ovulation and it will cease completely 14 days after. In the place of the corpus luteum a scar tissue is formed, the corpus albicans.

If fertilization does occur a new hormone will be synthesised, HCG, this hormone will replace the levels of LH equivalents to the luteal phase, allowing pregnancy to develop.

During the luteal phase the main ovarian product is progesterone.