Hormones of a Woman's Reproductive Cycle

Ovulation--the release of an ovum by the ovary--occurs only once at a moment in time during the cycle, even when more than one ovum is released.

The ovulatory mechanism also produces the two ovarian hormones, oestradiol and progesterone.

Oestradiol is produced alone by the developing follicle before ovulation; it stimulates the glands of the cervix to secrete a particular type of mucus ("mucus with fertile characteristics") which is essential for the sperm to pass through the cervix and reach the ovum. Oestradiol also stimulates growth of the endometrium lining the uterus (womb).

The cyclic changes in ovarian activity are controlled by the secretion of two hormones by the pituitary gland, follicle-stimulating hormone FSH) and luteinizing hormone LH).

The follicles within the ovaries have a threshold requirement for FSH below which no stimulation occurs.

There is also an intermediate level of FSH production which must be exceeded before a follicle is finally boosted into its full ovulatory response.

As the dominant follicle races towards ovulation, it produces rapidly increasing amounts of oestradiol. This oestradiol stimulates production of cervical mucus and also suppresses FSH production.

The fall in FSH also turns on a maturing mechanism within the dominant follicle which renders it receptive to the second pituitary gonadotrophin, LH. This surge of LH is the trigger which initiates the rupture of the follicle (ovulation) approximately 37 hours after the beginning of the surge of LH or 17 hours after its peak.

The maximum symptoms of the production of mucus with fertile signs are observed on the day of peak oestradiol production which precedes the Peak mucus symptom and ovulation. The rapid change following the Peak mucus symptom occurs very close to the day of ovulation and is due to the increasing production of progesterone at this time. The onset of the next menstrual bleed in the absence of pregnancy is highly predictable from these events.

After ovulation, progesterone and oestradiol are produced by the corpus luteum which forms from the ruptured follicle. This progesterone causes the abrupt change in the mucus which occurs immediately after ovulation and defines the Peak symptom.

Progesterone also prepares the oestrogen-primed endometrium for implantation of the fertilized ovum. 

In the absence of pregnancy, production of oestradiol and progesterone begins to decline approximately 7 days after ovulation and this results in shedding of the endometrium as menstrual bleeding 11-16 days after ovulation.

The Billings Method utilises the changes in cervical mucus production as observed by the woman herself for identifying the underlying events of the ovulatory cycle.