Charting the Course
October is the best month
to start charting.
Here's what you'll find.
On basal body temperature, the progesterone it's actually tracking, and why the map your body has been drawing all along is worth reading.
There's something quietly radical about taking your temperature before you get out of bed. Not because it's a hack, or because it produces data you can show to a nutritionist. But because of what happens when you do it consistently, over weeks and months: a pattern emerges that was there all along.
Basal body temperature — BBT — is the lowest temperature your body reaches during rest. In the context of the menstrual cycle, it tracks a specific and measurable event: the rise of progesterone after ovulation. Progesterone is thermogenic. It raises your resting temperature by approximately 0.2 to 0.5 degrees Celsius in the second half of your cycle — reliably, consistently, every cycle in which ovulation occurs. That thermal shift is your body announcing that ovulation has happened and the luteal phase has begun.
That's the map. Your body has been drawing it every month. Most of us were just never taught to read it.
Why October specifically
The argument for starting in October isn't sentimental — it's practical. Autumn regularises. Sleep schedules that drifted through summer begin to consolidate. You wake at more consistent times. You're less likely to be recovering from disrupted nights, festivals, or the erratic alarm patterns of August. BBT is notoriously sensitive to disruption — a late night, a fever, an alarm at the wrong phase of your sleep cycle can produce a reading that doesn't reflect your actual hormonal state — so the stability that October brings to daily routine is a genuine advantage for the quality of the data you collect.
The shortening photoperiod also helps. As melatonin windows lengthen and sleep architecture tends to deepen in autumn, overnight core temperature regulation becomes more stable. The chart you build in October is likely to be cleaner, more legible, and more informative than the chart you'd have built in July.
What the temperature is actually tracking
The biphasic chart — the pattern you're looking for — has a low phase and a high phase, with a clear thermal shift marking the boundary. Before ovulation, temperature sits in a lower range, typically 36.1–36.5°C. In the 24–48 hours around ovulation, it may dip slightly — not everyone sees this dip, and its absence doesn't mean ovulation didn't occur. Then, as progesterone rises from the newly formed corpus luteum, temperature climbs. It stays elevated throughout the luteal phase, typically 36.4–37.0°C, and falls back to the low range 1–2 days before menstruation begins — or on day one itself.¹
What you're tracking is progesterone. Not directly, but through its thermogenic effect on the hypothalamus. This means BBT is, among other things, a proxy for luteal quality. A short high phase — fewer than ten days — may suggest luteal phase insufficiency. A slow, gradual rise rather than a clear shift may suggest that progesterone is rising, but sluggishly. A monophasic chart — no clear shift at all — may suggest that ovulation didn't occur that cycle.
These aren't diagnoses. They're questions. But they are questions that a chart can help you form — and that far too many women never get the chance to ask because nobody gave them the map.
Touch any segment of the clock to read what your body is doing on that day.
The thermal shift is confirmation, not prediction
This is the distinction the BBT conversation sometimes loses. You can't use temperature alone to predict when you're about to ovulate in time to act on it — the shift happens after the LH surge, after the follicle ruptures. What temperature gives you is retrospective confirmation: yes, ovulation occurred, and here is the day the luteal phase began.⁴
This is still enormously useful. It tells you whether you are ovulating at all. It tells you the precise length of your luteal phase — something that is rarely measured in conventional care despite its clear relevance to cycle health, fertility, and hormonal wellbeing. And it tells you, over several cycles, whether your pattern is consistent or variable, which is information that is difficult to obtain any other way without continuous hormone monitoring.
Over time, the chart also responds to things beyond ovulation: travel, alcohol, infection, iron deficiency, chronic stress. Cortisol suppresses progesterone synthesis, which can blunt or delay the thermal shift. Consistently low temperatures across both phases can be a prompt to investigate thyroid function, particularly in its subclinical forms.² Your temperature is not just a fertility signal. It's a multi-dimensional biological readout that you happen to be able to collect at home, for free, with a two-decimal-point thermometer and three minutes of patience each morning.
Enter your last period date above to find out where you are right now.
What else BBT reveals
There's a broader story here, and it took me a while to find it in the literature.
Ovulation is not just a fertility event. Research by Prior and colleagues demonstrated that ovulatory cycles — cycles in which progesterone rises sufficiently to produce a clear thermal shift — are protective of bone density, cardiovascular health, and thyroid function in ways that anovulatory cycles are not.³ The progesterone that BBT is proxying is doing significant work beyond reproduction. When you can't see that shift on your chart — or when the shift is there but the high phase is short, unstable, or slow to rise — you're looking at a signal about more than just your next period.
Which is why the pattern decoder below matters more than it might initially appear. Understanding what different BBT curves communicate is the first step in being able to ask the right questions — to a practitioner, to yourself, to anyone who is supposed to be monitoring your health.
Why you weren't taught this
BBT charting exists in an uncomfortable middle ground: too low-tech to attract pharmaceutical interest, too female-specific to have attracted significant research funding, and historically associated with Catholic natural family planning in a way that put secular medical practitioners off engaging with it seriously for decades.
The Fertility Awareness Method, the Marquette Model, the Creighton Model — all have been dismissed in mainstream gynaecology as unreliable, a characterisation that conflates poorly-used methods with the underlying biology. The thermal shift itself is not a method. It is a physiological event. The evidence for its relationship to progesterone and ovulation is not disputed.¹ What is disputed is whether it is worth teaching women to read.
We should not need to be religious, trying to conceive, or enrolled in a clinical trial to have access to information about what our own temperature is recording. That this framing has persisted as long as it has is not an accident. It is a choice about whose knowledge is considered medically legitimate and whose isn't.
How to actually do this
A few things that make charting genuinely work, rather than producing noisy data you can't interpret:
- Use a basal thermometer, not a standard one. A standard fever thermometer reads to one decimal place. A basal thermometer reads to two (e.g., 36.23°C rather than 36.2°C). The thermal shift you're looking for is 0.2–0.5°C — readable on either, but far more clearly visible with two decimal places. They cost roughly €8–15.
- Take it before you move — before you speak, before you check your phone. BBT rises rapidly once you get out of bed and becomes useless as a basal reading. Keep the thermometer on your bedside table. Take it at the same time each morning within about 30 minutes. Even mild movement before measuring can shift the reading.
- Note disruptions — don't discard the data. A late night, alcohol, illness, waking earlier than usual — these shift the reading and shouldn't be deleted. Instead, note them on the chart and don't use those data points to identify the thermal shift. The chart tells a more honest story when disruptions are visible alongside the pattern.
- Look for the shift retrospectively, not day by day. You need at least three consecutive higher temperatures, all above the highest of the preceding six low temperatures, to confirm a thermal shift. This sounds like a rule from a 1970s manual because it is — but it remains the most reliable method for identifying the biphasic pattern without false positives.
- Track alongside something else. BBT is most informative when charted alongside at least one other fertility sign — cervical mucus, LH strips, cycle length. Temperature alone gives you the luteal phase. Combined with other signs, it gives you the whole picture: follicular development, the fertile window, ovulation confirmation, and luteal quality. October is also when cycle tracking apps sometimes prompt you to buy things you don't need. A paper chart or a spreadsheet works just as well.
Your body has been keeping this record for years. Every cycle, a map has been drawn. The low phase, the thermal shift, the sustained high, the slow pre-menstrual descent. Month after month, regardless of whether anyone was looking.
October is a good time to start looking. The mornings are darker and slower. Sleep is more regular. The chart will be cleaner than it would have been in August, and the season's natural rhythms will be working in your favour rather than against you.
You don't need an app subscription. You need three minutes each morning, a basal thermometer, and several weeks of patience. What you find — the shape of your own curve, the timing of your shift, the length of your luteal phase — is information that belongs to you, that your body has been generating all along, and that no one had any particularly good reason not to share.
– Nina- Su, H.W., Yi, Y.C., Wei, T.Y., Chang, T.C., & Cheng, C.M. (2017). Detection of ovulation, a review of currently available methods. Bioengineering & Translational Medicine, 2(3), 238–246. doi:10.1002/btm2.10058
- Guida, M., Tommaselli, G.A., Palomba, S., Pellicano, M., Moccia, G., Di Carlo, C., & Nappi, C. (1999). Efficacy of methods for determining ovulation in a natural family planning program. Fertility & Sterility, 72(5), 900–904. doi:10.1016/S0015-0282(99)00349-5
- Prior, J.C., Vigna, Y.M., Schechter, M.T., & Burgess, A.E. (1990). Spinal bone loss and ovulatory disturbances. New England Journal of Medicine, 323(18), 1221–1227. doi:10.1056/NEJM199011013231801
- Wilcox, A.J., Weinberg, C.R., & Baird, D.D. (1995). Timing of sexual intercourse in relation to ovulation. New England Journal of Medicine, 333(23), 1517–1521. doi:10.1056/NEJM199512073332301
- Vigil, P., Cortés, M.E., Zuniga, A., Riquelme, J., & Ceric, F. (2017). Ovulation, a sign of health. The Linacre Quarterly, 84(4), 343–355. doi:10.1080/00243639.2017.1394053