In the bedside drawer there is a thermometer and a few sheets of graph paper with curled corners. Every morning, as soon as the alarm goes off, she lies still, takes her temperature, then marks a dot on that month’s sheet. Three months go by, the dots join into a jagged line going up and down, and she brings all three sheets to her appointment with one question: in which months did she ovulate, and on which day? Part of the answer is right there on those sheets.
Count the cycle length from the first day of your period
Day one of the cycle is the first day you see your period, and the cycle lasts until the day before your next period. The definition system of the International Federation of Gynecology and Obstetrics (FIGO; Munro et al., 2018) classes cycles lasting 24 to 38 days as normal.
Old traditional medicine texts called the period the monthly message (“the monthly message”, 月信, yuè xìn), news that arrives each month. That news rarely comes at exactly 28 days. Fehring and colleagues (2006) followed 141 healthy women with regular periods over 1,060 cycles. The average length was 28.9 days, 95% of cycles fell within 22–36 days, and 42.5% of the women had long months and short months that differed by more than 7 days. That 22–36 day range was measured in a group of 141 people, while 24–38 days is FIGO’s clinical threshold. Bull and colleagues (2019) analyzed more than 600,000 cycles from a period-tracking app, with an average of 29.3 days.
Ovulation day shifts mainly in the first half of the cycle
Each cycle has two phases. The follicular phase runs from the first day of your period until ovulation, and the luteal phase from after ovulation until just before the next period. In Bull’s data, the follicular phase averaged 16.9 days but varied widely, from 10 to 30 days in 95% of cycles; the luteal phase averaged 12.4 days, mostly within 7–17 days.
So ovulation does not happen on a fixed day 14. Fehring found that only 25% of women had all six of their fertile days fall within days 10–17 of the cycle. The fertile window (Vietnamese) is about six days long and ends on the day of ovulation. Counting days alone makes it hard to know where those six days fall.
Vaginal discharge, on the other hand, gives advance notice: in the days just before ovulation it is thin, clear, slippery and wet all day, then it lessens, thickens or becomes almost dry, so all you need is to write one word each day, “dry”, “moist” or “slippery-wet”, next to the calendar square, and the slippery-wet days are usually the days you are most likely to conceive.
Taking your basal body temperature every morning
Basal body temperature (BBT) is your body temperature at complete rest, taken right after you wake up. For the readings to be comparable with each other:
- Take it at the same time window each morning, before you sit up, drink water or go to the bathroom;
- Use one thermometer and one measuring site for the whole cycle;
- Record it on the chart right away, and mark the day your period starts;
- Make a note of nights with little sleep, days with fever, drinking alcohol or traveling, because temperatures on those days are easily thrown off.
A typical chart has two levels. The first half of the cycle is lower; after ovulation, the temperature rises slightly and stays at the higher level until just before your period. From day to day it can still move up or down a little.
BBT only confirms ovulation after it has happened
Temperature rises only after ovulation, so by the time the line on the chart steps up to the higher level, the most fertile days of that month have almost passed. Guermandi and colleagues (2001) used transvaginal ultrasound as the reference standard in 101 women with difficulty conceiving. The urinary LH surge (the hormone signal that ovulation is coming) always appeared before the follicle ruptured and matched the ultrasound in 97% (98 of 101 women); the BBT chart matched in only 74%. The lowest point on the chart, often called the day of ovulation, predicts the day of ovulation poorly. Bull’s data came from users of one app, and Guermandi’s group had only 101 women with difficulty conceiving, so the results may not apply to everyone.
For the question from the start of this article, the chart can tell whether she ovulated that month and roughly when in the cycle, but it does not point to one exact day. Putting several months side by side shows which day the temperature usually rises around, how long the high stretch lasts, and which months do not show two clear levels.
If taking your temperature every morning stresses you out, tell your doctor. LH test strips give advance notice that ovulation is about to happen; follicle-tracking ultrasound or a single blood progesterone test in the middle of the luteal phase also shows whether you ovulated (Guermandi et al., 2001).
When to bring your chart to a doctor
Bring your period calendar and chart to an appointment when:
- Your cycle is shorter than 24 days or longer than 38 days, month after month;
- You miss a period in some months, or your periods come further and further apart;
- For several cycles in a row, the chart shows no clear two levels of temperature;
- Your period comes only a few days after the temperature rises, meaning the high stretch is shorter than the 7–17 days the post-ovulation phase usually lasts according to Bull’s data;
- Your vaginal discharge has an unusual color, a smell or causes itching; in that case, don’t read your discharge the way described above.
If your temperature stays high for longer than usual and your period is late, take a pregnancy test first. If you are taking any medicines, tell your doctor.
When to see a doctor if you are trying to conceive and have no good news yet, and why both partners should get checked at the same time, is covered in our article trying to conceive: when to see a doctor.
How the chart is used at our clinic
Patients in treatment can enter their temperature each morning in an online form the clinic sends them, and the chart is built automatically so Dr. Tam can review it before each appointment. Dr. Pham Tran Dan Tam (MD, Specialist Level I in Traditional Medicine) uses the BBT chart to follow the temperature in each phase of the cycle, helping patients understand their own bodies and helping the doctor follow treatment results month by month. At the clinic, the chart is also read according to the chart-type classification of Professor Xia Guicheng (夏桂成) to choose the direction of the herbal treatment for each phase of the cycle; this is clinical experience and has not yet been tested by modern research.
The chart does not tell you why conceiving is taking longer; that requires ultrasound and tests for both wife and husband.
Worth keeping
- Write down the start date of each period, and wait until you have three months or more before working out how long your cycle is;
- Keep the thermometer by your bed, take your temperature before you get up, at the same time window each morning, and use the same thermometer for the whole cycle;
- Next to each temperature reading, add one word about that day’s discharge;
- Read the chart across several months together; don’t use it to pin down a day within the month you are measuring;
- Your charts and period calendar are what the doctor needs to see at every appointment.
If you have been charting for a few months and don’t know where to start reading, or your cycle falls outside the 24–38 day range, Dr. Pham Tran Dan Tam will go through each chart with you and discuss the next step.
References
- Munro MG, Critchley HOD, Fraser IS; FIGO Menstrual Disorders Committee (2018). The two FIGO systems for normal and abnormal uterine bleeding symptoms and classification of causes of abnormal uterine bleeding in the reproductive years: 2018 revisions. Int J Gynaecol Obstet 143(3):393-408. https://doi.org/10.1002/ijgo.12666;
- Fehring RJ, Schneider M, Raviele K (2006). Variability in the phases of the menstrual cycle. J Obstet Gynecol Neonatal Nurs 35(3):376-84. https://doi.org/10.1111/j.1552-6909.2006.00051.x;
- Bull JR, Rowland SP, Scherwitzl EB, Scherwitzl R, Danielsson KG, Harper J (2019). Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. NPJ Digit Med 2:83. https://doi.org/10.1038/s41746-019-0152-7;
- Guermandi E, Vegetti W, Bianchi MM, Uglietti A, Ragni G, Crosignani P (2001). Reliability of ovulation tests in infertile women. Obstet Gynecol 97(1):92-6. https://doi.org/10.1016/s0029-7844(00)01083-8.
