The thermometer

In the case of recording and comparing changes in the resting or basal temperature of the body a reliable thermometer is needed – the change is a subtle fluctuation and not one which can be discerned subjectively, as can fever. A mercury or digital thermometer can be used. Digital thermometers are a little easier to read, especially in those groggy first few minutes of the day just after waking, but they are more expensive than the mercury type, and they can be slightly less accurate (although this is not usually a problem for our purposes). It is possible to buy a mercury thermometer designed to measure only the basal body temperature (called a basal body temperature thermometer). In this case the scale covers a narrower range than usual and the gradations are more spread out and easier to read than on a regular thermometer. However, most women use the regular thermometer normally kept in the bathroom cupboard for measuring fevers. Now, however, the thermometer will be kept on the bedside table.

On waking in the morning, after at the very least 3 h uninterrupted sleep, the thermometer is placed in the mouth and left for 3–5 min. BBT can also be measured by placing the thermometer in the vagina or rectum. Because these routes are slightly less convenient, they are less often used. However, if we are finding that the temperature readings are very erratic and it is hard to determine a pattern, then I sometimes recommend the vaginal route, which can give a more stable pattern.

The thermometer is removed, the temperature reading taken and its value noted on the chart under the correct day and date. For a mercury thermometer, it can be placed carefully on the bedside table and the reading taken some time later in the morning or even when going to bed the next evening. This is because most mercury thermometers will remain stable at the temperature they have reached until the mercury is given a firm shake down with a sharp flick of the wrist. After reading and recording the temperature, the mercury in the thermometer should be returned to its base level and the thermometer wiped clean ready for the next morning’s use.

It is important that the temperature is read at roughly the same time each morning, because later waking raises the basal body temperature. Thus, a very erratic-looking chart may reflect nothing more than a rather erratic sleep and waking pattern. Some studies have shown that the basal temperature rises 0.09°C for each hour of delayed rising in the morning.⁶ Thus, BBT charts can be adjusted for different waking times by moving the recorded temperature one square up or down for each hour of waking earlier or later than usual.

The urinary ovulation predictor kit

These kits can be used to test for the rise in luteinizing hormone (LH) prior to ovulation, i.e., at the later stages of the follicular phase. LH produced by the pituitary as the dominant follicle ripens usually does not last very long in circulation and so these kits test for a form of LH that has an extended life span due to transformation in the urine. It is recommended that the urine is tested during the day (not first thing in the morning) after avoiding liquids for about 2 h (so that the urine is not too dilute). A positive reading will show two colored lines with the test line being equal to or darker than the control line. This indicates the LH levels have risen and that an egg will probably be released within 12–48 h. Using the predictor kit in conjunction with the BBT is a useful strategy, especially if the fertile mucus signs are not pronounced. It gives a reliable guide to the most fertile days of the cycle and the best days to be having sex.

The chart

Many different variations of the BBT chart have been developed over the years, initially by those using BBT as a contraceptive tool. Basically, all charts record the basal body temperature and compare the values over the entire menstrual cycle. As other kinds of fertility signals were described (e.g., cervical fluid and position) by the Billings’, then provision was made for these to be recorded alongside the temperature readings. Room for other items of personal interest, such as dates of sexual intercourse, or late nights or the occurrence of relevant symptoms has also been added.

Each day the temperature is recorded on the chart, along with other observations related to fluctuations in the hormones (Fig. 3.2), e.g., changes in breast and nipple tenderness or swelling may be noticed or changes in abdomen bloating or discomfort, in sleep patterns, in moods, in food cravings and, most importantly, changes in vaginal lubrication and discharge (this latter can also include spotting of blood).

Fertile mucus

The changes in vaginal discharge which flag changes in fertility are determined both by subjective feelings of moisture at the vulva and by more objective observation of the discharge collected on underclothes, toilet tissue or fingers.

The estrogen peak, which stimulates the production of fertile mucus, usually starts on average 6 days before ovulation. Four different types of mucus are produced by specialized glands in the cervix in response to this estrogen (Fig. 3.3).⁷ The cervical discharge produced at infertile times (i.e., at all times except ovulation) is associated with a dry feeling at the vulva. It is called G-type mucus and is thick, pasty and impenetrable. It is produced in crypts of the cervix at the end of the cervical os nearest the vagina (Fig. 3.4). As estrogen levels begin to rise, the cervix produces, from crypts a little higher in the cervix, the more liquid L-type of mucus. This causes the vaginal sensation to be more sticky or wet. As ovulation approaches, crypts still higher in the cervical os produce mucus which is more stretchy and slippery. It is this S-type mucus that is sometimes referred to as the egg-white-like mucus. The sensation at the vulva now is distinctly wet and strings of S mucus, sometimes mixed with clumps of L mucus, may be noticed. The phenomenon which gives this mucus its elasticity is called spinnbarkeit, or ‘spinn’ for short.

Finally, as ovulation is imminent, the mucus loses its stretch as the cervix produces its P mucus from the very top of the cervical canal. P mucus is so named for its rich potassium content. While this final secretion is less thick and stretchy, it is extremely lubricative and produces a slippery sensation in the vulva. The last day that P mucus is produced is the most fertile day of the whole cycle, i.e., the day before the egg is released or the day it is released.

Fertile mucus forms graphic ferning patterns when it is dried on a smooth surface such as a glass microscope slide. The three different types of fertile mucus crystallize in three distinct patterns: the L mucus makes intricate and many branched ferns; the S mucus makes delicate pine needles lined up close and parallel; and the P mucus makes hexagons (Fig. 3.5).

The function of the branched L mucus is to catch and filter out some of the abnormal or poor-quality sperm before they reach even the uterus. The S mucus, on the other hand, provides a rapid sperm transit system, as it creates clear pathways for them to swim upwards to the uterus. Because it is composed of mucin strands lined up longitudinally it has the unusual quality of being particularly stretchy and adhesive to itself. For example if you tried to pour fertile mucus from one cup to another you would have to cut the flow with scissors if you wanted to stop it halfway.

The P mucus appears to have some quality which gives sperms a final boost or activation as they climb through the cervix to the uterus.

A summary of fertile mucus characteristics is given in Table 3.1.

One easy way to evaluate quality and quantity of cervical secretions is to collect a little from the vaginal entrance (if it is plentiful enough) or gently from the cervical surface itself (if it is less plentiful) with clean fingers and see how much elasticity there is as the fingers are drawn apart (Fig. 3.6). Some women prefer to use toilet tissue to collect the fluid or rely on sensations of moisture alone. Some teachers of the Billings’ method stress learning to interpret the subjective sensations of dryness or lubrication at the vulva without further investigations using the fingers (presumably to avoid introducing infection to the vagina or for cultural reasons when this method is taught in developing countries).

The importance of recognizing the changes in the fertile mucus lies in the fact that it precedes ovulation and therefore alerts a woman to her most fertile days. The last day of any fertile mucus, whether seen as a secretion from the vagina or felt as a moist sensation, is often referred to as the ‘peak day’. This refers to the day of peak fertility, the day before or the day the egg is released. Nature’s design for conception is not random at all but has designed very exact timing. Producing the fertile mucus to facilitate the sperm’s journey up the cervix in the days before ovulation is aimed at getting these sperm inside the female reproductive tract well before the egg is released (Fig. 3.7). This is because this largest single cell in the body is one of the shortest lived, being fertilizable for only 6–12 h. Some of the millions of lively little sperm cells, on the other hand, have been known to survive for 5 days in the female reproductive tract, although they are most able to fertilize the egg in the first 48 h after delivery to the vagina. So, ideally, there will be a bunch of some hundreds of eager sperm that have survived the hazardous journey as far as the fallopian tubes, lying in wait for the egg as she is launched. Some studies have indicated that the day for sexual intercourse which then leads to the highest conception rate is, in fact, 2 days before ovulation.