Normal menstrual cycle represents a coordinated serial event, repeated month by month, at regular intervals, in which the hypothalamus participates along with the secretion of GnRH, the pituitary gland secreting follicle stimulating hormone and luteinizing hormone (LH), and the ovary which responds to those hormones, recruiting a dominant follicle and secreting estradiol and inhibin A. Estradiol stimulates endometrial proliferation and production of cervix mucus. A peak of estradiol triggers discharge of LH, responsible for ovulation and posterior secretion of progesterone by the corpus luteum, which in turn, involutionates 14 days later if it does not receive the stimulation of hCG (pregnancy). Normal menstrual cycles last 28 ± 7 days, being accepted a fluctuation of ±2 days in the same woman, as a normal pattern, what is described as a regular cycle. Normality of these events would allow to achieve a successful embrionary implantation in the case of looking for pregnancy. For this it is required that an adequate ovule to be fertilized is reached by a capacitated spermatozoon, during the ovulatory stage. Spermatozoon can survive as long as 5 days at feminine genital tractum, but the ovum is possible to be fecundated only during 12–24 hours. Fecundation occurs at the distal third of the fallopian tube and the fecundated zygote arrives in the state of a morula, to be implanted at the endometrium 4 days later. Once the state of blastocyst is reached, it is detached from its shaggy area (hatching) and it is implanted in a receptive endometrium when the window of implantation is open (days 7–9) postovulation. The first marker of pregnancy is the detection in maternal blood of β-hCG. No more than the 25% of fertile couples exposed to pregnancy can achieve gestation at the month of exposure.
Part of the book: Menstrual Cycle