A total of 437,577 unique users and 896,051 unique, normal menstrual cycles met inclusion criteria with an average of 2.05 ± 1.8 cycles per user (Fig. 2). The mean age of users was 30.0 ± 5.4 years; users under age 18 or over 45 were excluded from the analysis. Users were included from 222 countries, with 113 countries having at least 100 users. The largest group of users was in the United States (n = 126,352), followed by Germany (n = 45,018) and the United Kingdom (n = 35,007).
Of the 437,577 unique users included in this analysis (last cycle), the mean cycle length was 28.4 ± 2.6 days (median 28.0 days), and the mean length of bleeding was 4.0 ± 1.1 days (median 4.0 days). Most included cycles had a length of 25–31 days (75.7%, n = 331,360), followed by a length of ≥ 32 days (15.8%, n = 69,479) and < 25 days (8.4%, n = 36,738). Calculated from the predicted day of ovulation, the mean follicular phase length amongst users was 14.4 ± 3.0 days (median 14.0 days), and the mean luteal phase length was 14.0 ± 2.7 days (median 14.0 days).
Of the 896,051 unique cycles, somatic symptoms were reported most often; 88.3% (n = 790,923) of cycles reported at least one somatic symptom. Comparatively, gastrointestinal symptoms and discharge symptoms were reported in 60.8% of cycles each (n = 545,107 and 544,460, respectively), negative mood symptoms were reported in 43.3% (n = 388,249) of cycles, and positive mood symptoms were reported in 35.4% (n = 317,558). The type and frequency of symptoms are shown in Fig. 3. Menstrual cramps, breast tenderness, headache, and fatigue were the most common symptoms reported.
Symptoms were next assessed by the menstrual cycle phase. Somatic, gastrointestinal, and negative mood symptoms were most frequent in the late luteal phase and least frequent in the mid-follicular phase (Fig. 4). 57.3% of all logged instances of tender breasts occurred during the late luteal phase compared to 1.8% in the mid-follicular phase. Discharge and positive mood symptoms were most common during the fertile window. 63.1% of all logged instances of egg white discharge and 45.3% of all logged instances of high sex drive were during the fertile window.
For cycles between 25–30 days in length (n = 570,123 cycles), we examined daily symptom logs for the three symptoms with the most variability per category (Fig. 5). Overall, most symptoms are reported on day 1 of the cycle (436,812 symptoms logged from 211,021 cycles out of 570,123 total cycles); however, the fertile window is where most cycles have at least one symptom logged. Out of 570,123 total cycles, 382,115 cycles had at least one symptom logged during the fertile window, and 283,652 had at least one symptom logged during the bleeding days. The prevalence of somatic, gastrointestinal, and negative mood symptoms are highest on day 1 but decline quickly, reaching their lowest point around day 7 before slowly increasing throughout the rest of the cycle. For positive mood symptoms, the prevalence of energetic mood remains relatively stable throughout the cycle. In contrast, happy and high-sex-drive moods increase slowly in the early part of the cycle, peaking around day 14 of the cycle and reaching baseline around day 20. Discharge symptoms are rarely logged during the early part of the cycle where bleeding occurs. Egg white discharge increases rapidly between days 6 and 13 before decreasing quickly to a low level by day 20.
For countries with at least 100 users, the number per country ranged from 103 in The Bahamas to 126,352 in the United States (Supplemental Table 1). The prevalence of cramps during the last cycle ranged from 30.5% in Vietnam to 74.6% in Iceland (Fig. 6). Bloating ranged from 17.6 in Thailand to 49.7% in Chile. Mood swings ranged from 13.3% in Hong Kong to 43.1% in Iraq.
Mood symptoms were most highly correlated with each other. The anxious mood was highly correlated with obsessive thoughts (r = 0.60) and depressed mood (r = 0.58). Depressed mood was correlated with obsessive thoughts (r = 0.55), and happy mood was highly correlated with energetic mood (r = 0.62, Fig. 7). Similarly, fatigue was highly correlated with somatic and gastrointestinal symptoms such as bloating (r = 0.62), headache (r = 0.60), backache (r = 0.58), and nausea (r = 0.55).
A term frequency-inverse document frequency (tf-idf) analysis by cycle phase showed how pairs of symptoms correlate within cycle phases, adjusting for the frequency of individual symptoms (Fig. 8 and Supplementary Tables 2 and 3). During the bleeding days and late luteal phase when cramps are common, the top five tf-idf values all included cramps, and cramps were most closely associated with nausea (tf-idf = 54,861 bleeding days, tf-idf = 68,467 late luteal). Happy and energetic were the top pair in the mid-follicular phase (tf-idf = 23,333) and second in the fertile window (tf-idf = 55,636).