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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