Giddy, ignore my last post.
Here's a taster: BBT and the Fertility Window
'BBT is the waking temperature of the body before any activity. It reflects the ovarian cycle in two ways. Within 1 to 2 days before the LH surge there is a nadir (low point) in BBT (Martinez et al., 1992). For over 30 years, this nadir in temperature has been identified as possibly useful in predicting ovulation (Lundy et al., 1974). Following ovulation, women generally experience an increase in the BBT of 0.5°F to 1.0°F; this is called a biphasic pattern (Figure 2). This increase is thought to be due to the thermogenic effect of pregnanediol, a metabolite of progesterone, which increases after ovulation and is secreted by the corpus luteum. The biphasic shift can therefore be used as a confirmatory marker of ovulation. Advising women to use BBT basically alerts them to this small increase in body temperature, indicating that ovulation has occurred. However, it
has long been recognized that some women may ovulate without a clear rise in temperature; this is called a monophasic pattern (Figure 3) (Morris, Underwood, & Easterling, 1976). This makes the use of BBT as a method of timing intercourse to achieve pregnancy less than useful.
Studies primarily from researchers at the National Institutes of Environmental Health Sciences have provided evidence that there is essentially a 6-day interval of fertility ending with the day of ovulation (Wilcox, Weinberg, & Baird, 1995). They also have shown that the most
fertile day is the day before ovulation and that the timing of the 6-day interval varies across menstrual cycles (Dunson, Baird, & Wilcox, 1999; Wilcox, Dunson, & Baird, 2001). The probability of pregnancy decreases by about 50% during this 6-day window when cervical mucus is not observed 
(Dunson, Sinai, & Colombo, 2001). Pregnancy probabilities are highest when vulvar observations indicate the presence of the most fertile type of estrogenic cervical mucus [ewcm] (Bigelow et al., 2004; Stanford, Smith, & Dunson, 2003).'