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Ovulation - the release of an ovum by the ovary and therefore the
only time during the cycle when the ovum is exposed for fertilization--is
the central event of the fertile ovarian cycle. It determines the
time when pregnancy can occur from an act of intercourse which is
the period of 3-4 days (rarely 5-6 days depending on the cervical
mucus) before ovulation determined by the fertilizing life span
of the sperm and up to 24 hours after ovulation determined by the
fertilizable life span of the ovum. Outside this time period a woman
cannot conceive from an act of intercourse no matter how hard she
tries. Even within this time period, pregnancy from an act of intercourse
is not a certainty, the chances vary depending on the couple and
the timing of intercourse in relation to ovulation. Maximum fertility
is reached during the period of 24 hours before ovulation and several
hours afterwards. If the chances of pregnancy at this time are 70%
per cycle, it takes two cycles for 90% of couples having intercourse
on the most fertile day to achieve pregnancy. If the chances at
the beginning of the fertile period are 10% per cycle, it takes
24 cycles for 90% of couples having intercourse at this time to
achieve pregnancy. Many authorities would say that the chances are
much less than the figures given. Even with in vitro fertilization
(IVF), which many would like to think is the ultimate in assisted
pregnancy, most health funds allow up to six cycles of treatment.
Thus, couples who break the rules of the Billings Ovulation Method
(BOM) and do not become pregnant should not conclude that the rules
do not apply to them, chance has been on their side. Alternatively,
if they have had intercourse on the most fertile day they should
not expect pregnancy to follow as a certainty. In animals, Nature
has ensured a maximum fertilization rate (but not 100%) by restricting
intercourse to the most fertile day of their cycle by the phenomenon
of oestrus. Thus the assessment of ovarian activity and the accurate
timing of ovulation are basic requirements in natural family planning
(NFP) for avoiding pregnancy and under all circumstances including
IVF for achieving it.
There are six main methods by which ovarian activity can be monitored
and the time of ovulation determined.
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.
3. The LH Surge
The surge in LH production by the pituitary gland triggers ovulation
which occurs approximately 36 hours after the beginning of the rise
in LH or 17 hours after its peak. Thus the time of ovulation can
be determined to within a few hours by either criteria. The LH peak
day is readily identified using home kits and, as it immediately
precedes the day of maximum fertility, it is commonly used for timing
intercourse for pregnancy achievement. To detect the beginning of
the LH rise requires more sensitive laboratory assays. However,
this was the procedure used in IVF for timing egg pickup in unstimulated
cycles because it provided an accurate 36-hour period for preparing
for the laparoscopy. Today it is usual to hyperstimulate the ovaries
to produce multiple follicles and eggs and then ovulation can be
induced at a prearranged time by giving the ovulating dose of HCG
36 hours beforehand. Nevertheless, ovulation does not necessarily
occur in the ovary following an LH surge from the pituitary, as
can be seen from the later section describing the continuum of ovarian
activity (see p. 17). Furthermore, ovulation has been documented
without an LH surge being identified, although some release of LH
must have occurred to trigger the ovulation. The rise in progesterone
output to reach a level which can be defined for the majority of
women is actually a more reliable marker of ovulation and a better
proof that there has been an LH surge and that the ovary has indeed
ovulated in response to it.
4. Basal Body temperature (BBT)
The rise in progesterone output at ovulation which causes the termination
of mucus production also causes a rise in basal body temperature
of approximately 0.3 degrees Centigrade. This rise is readily measured
and has been widely used for confirming that ovulation has occurred.
However, the rise in temperature in relation to the changes in progesterone
levels is very variable so that the timing of ovulation by the temperature
shift can be in error by up to -1 to + 4 days. The information is
retrospective and is of no value in predicting ovulation. The Sympto-thermal
methods of NFP include measurement of the BBT to determine that
ovulation has occurred and to calculate the beginning of the post-ovulatory
infertile phase. The BOM considers that measuring the BBT is unnecessary
and that the progesterone change in mucus production (PC) provides
all the necessary information.
5. Measurement of Oestrogen and Progesterone
Output
By observing the changes in the vaginal discharges the BOM effectively
measures the cyclic changes in oestrogen and progesterone output
by the ovaries. Oestrogen and progesterone levels can be measured
in blood by radioimmunoassay or their metabolites can be measured
in urine. Blood assays are widely used but have the disadvantage
that the stress of daily sampling, which is necessary to provide
the complete picture of ovarian activity around ovulation, can inhibit
ovulation. Most of the validation of the BOM has been done using
urine assays. Women have no difficulty in collecting urine daily
(3 hour collection) and the assays have been simplified to the stage
that women themselves can do accurate testing at home (using the
Home Ovarian Monitor). This device is used in many BOM centres to
help women who need reassurance that they are interpreting their
symptoms correctly and it has many applications in assisted reproduction
and in further research.
6. Ultrasound Scanning
The growth of follicles, rupture of a follicle (ovulation) and
development of a corpus luteum can be visualized by ultrasound scanning.
In fact, the actual rupture of the follicle, the extrusion of the
ovum and follicular fluid, the blood supply to these structures
and the degree of stimulation of the uterine endometrium as a result
of the hormones produced, can all be readily seen. This is thus
the most accurate method of timing ovulation. Ultrasound scanning
has played an important role in providing basic information on all
phases of ovarian activity, and its agreement with the findings
based on the hormone patterns and mucus symptoms has added much
to the confidence we have in the rules of the BOM. For daily application,
ultrasound scanning is expensive and therefore it is usual to assess
ovarian activity by another method and use ultrasound scanning as
the final confirmation that ovulation is imminent.
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