1. Vaginal bleeding

Every women is taught to document her bleeding pattern and this is the method used to assess ovarian activity since the human race began. Onset of vaginal bleeding is used to mark menarche and its cessation marks the menopause. Pregnancy is indicated when regular menstruation stops abruptly. Physiological bleeding is the result of shedding of the lining of the body of the uterus (the endometrium) after stimulation by the hormones oestrogen and progesterone produced by the ovaries during ovarian activity. It usually results from withdrawal of oestrogen and progesterone activity at the end of an ovulatory cycle. Such bleeding is called menstruation. Bleeding can also be the result of oestrogen activity alone produced by an ovarian follicle which has not ovulated. This is called anovulatory bleeding.

Bleeding is the end result of the ovarian activity, it gives little information about the ovarian events which have preceded it and it occurs at variable levels of hormone withdrawal. In an ovulatory cycle, the time of ovulation can be calculated as occurring 11-16 days before the onset of the following menstruation. Women with menstrual cycles which are regular enough for the date of menstruation to be predictable can also predict the date of ovulation by this calculation. This is the basis of the rhythm calculations which were used in the earliest methods of NFP. However, no woman is completely regular for the whole of her reproductive life and, even for the most regular women, errors in the calculations eventually occur, particularly during times of stress, lactation and approach of menopause.

2. Cervical Mucus and Related Vaginal Discharges

Oestrogen produced by the ovaries during ovarian activity causes production of mucus by the cervix and it also causes growth and shedding of the epithelial cells lining the vagina, the responses depending on the degree of ovarian activity and on the amounts of oestrogen being produced. These two sites are more sensitive to oestrogen action than the endometrium and the changes can be observed even when the ovarian activity and the oestrogen levels produced are insufficient to cause bleeding, such as in the lead up to menarche. These are the sources of the vaginal discharges utilized in assessing fertility and infertility by all of the modern methods of NFP including the BOM. The way the BOM does this is unique. While the woman is in the upright position such as when doing her normal daily activities, the vaginal discharges drain into the vulval areas and are felt as a sensation there which is either dry, sticky or progressing to slippery. The woman is continuously aware of these sensations without deliberately thinking about them or investigating them and, by understanding their significance, is in touch with the underlying ovarian activity and her fertility throughout the day. While the woman is asleep, the discharges do not drain away so that time for the woman to be in the upright position is required for the sensations to be appreciated. When ovarian activity is absent and no oestrogen is being produced there is usually no discharge and the feeling is one of dryness which persists throughout the period of inactivity ("dry basic infertile pattern" or "dry BIP"). Alternatively, a woman may experience a slight unchanging discharge at this time. This BIP is due to small amounts of mucus being shed from the mucus plug in the cervix. When a small amount of ovarian (follicular) activity is present but not progressive, oestrogen is produced in small and constant amounts and this causes a discharge which comes mainly from the vaginal epithelial cells ("BIP discharge"). Greater ovarian activity which still does not progress results in higher constant levels of oestrogen production which cause a small but constant production of mucus by the cervix. This third BIP is usually only seen during breast-feeding and the approach of menopause. Thus the three BIPs are the result of different levels of oestrogen production, the essential feature being that oestrogen production remains constant for a period of time. A change from one oestrogen level to another is recognizable by a change from one discharge to another, but the change and the new discharge do not progress ("do not go anywhere") and thus differ markedly from the changes in the oestrogen levels and the discharges seen during the progressive lead up to ovulation. During a BIP, once it is established that it is a BIP, the Early Day Rules for intercourse are followed.

Before ovulation can occur a follicle containing an egg must commence and complete its rapid growth phase and this causes marked changes in the vaginal discharges. The oestrogen output rises from a base line corresponding to minimal or absent follicular activity and increases during the rapid growth phase at a rate of approximately 1.5 times per day over a period of 5-6 days. This results in an immediate change from a BIP (the "oestrogen rise" or "ER") followed by a rapidly changing mucus pattern. Thus any change in the BIP can herald either the beginning of another BIP associated with another phase of infertility or, more usually, the beginning of the rapid growth phase of a follicle associated with the fertile phase of the cycle and impending ovulation. Therefore, a period of wait-and-see abstinence at this time is required to distinguish between the two possibilities. When a follicle is proceeding to ovulation, the increasing oestrogen production causes important changes in the cervical mucus which are listed elsewhere (mucus with fertile characteristics). The important feature of this mucus is that it is changing daily to more fertile characteristics in line with the rising oestrogen output of the growing follicle, an important final feature being a slippery sensation (lubrication). This progressive mucus symptom marks the fertile phase of the cycle. If the aim is pregnancy avoidance, the identification of the change from the preceding BIP (the ER) provides sufficient time to allow for the longest fertilizing life span of the sperm before the ovum is available for fertilization at ovulation.

The LH surge which initiates ovulation of the developed follicle, also causes the second ovarian hormone, progesterone, to be produced by the follicle. This production is small but significant at first and then increases rapidly after ovulation. This progesterone strongly reverses the action of oestrogen on the cervix and vaginal epithelium and causes the discharges to rapidly lose their fertile characteristics. This change due to progesterone (the "progesterone change" or "PC") is readily recognized. It is a very important symptom because it shows definitely that ovulation is occurring and is closely related in time to ovulation. With this knowledge, the remainder of the ovulatory cycle can be predicted with confidence. The BOM uses the term Peak day for the day of peak fertility and defines it as the last day of mucus with fertile characteristics (slippery) before the PC. The Peak day is not necessarily the day of maximum mucus production and it is not unusual for a woman to notice the slippery sensation in the morning and to follow the PC as it progresses during the day. In this case the Peak day and the PC occur on the same day. Ovulation occurs on the Peak day or the day of the PC, or occasionally on the next day. Thus the PC times ovulation to within ±24 hours. The rule of the BOM for calculating the end of the fertile period from the Peak day and entry into the post-ovulatory infertile phase (the Peak rule) allows for the range of this timing and for the fertilizable life span of the ovum. When this post-ovulatory infertile phase has been reached, pregnancy from an act of intercourse is impossible and, for pregnancy avoidance, all days are available for intercourse until the commencement of the next menstruation.

Thus the fertile phase of the cycle (the "window of fertility") can be recognized by beginning with the first change in the discharge from a BIP (the ER) progressing with fertile characteristics in line with the rising oestrogen production. Ovulation can be recognized by the progesterone change (the PC) and the end of the fertile phase can be calculated from this. The letters in the words "prompt day" summarize the events occurring on the day of the PC, i.e. progesterone rise, ovulation and mucus past, today. The BOM adds 3 days after the Peak day to be 100% certain that the post-ovulation infertile days have been reached.

Besides identifying the underlying ovarian activity and timing ovulation, cervical mucus with fertile characteristics is itself important for fertility, being necessary for maintaining the fertilizing capacity of the sperm and for their passage from the vagina through the cervix to the fallopian tubes. As menopause approaches, the ageing cervix may lose its responsiveness to oestrogen so that no mucus is observed even though ovulation is occurring. Such women are infertile. However, in this event, care in observation is required in case a brief discharge of mucus conferring a brief period of fertility is missed. Absent or poor mucus production before ovulation in a woman being investigated for infertility is often the cause of the infertility. It should be remembered that inhibition of cervical mucus production is an important point of action of the contraceptive pill through the progestogen it contains.

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