Prospective 1-year assessment of within-woman variability of follicular and luteal phase lengths in healthy women prescreened to have normal menstrual cycle and luteal phase lengths

Abstract

Study question: What is the relative length variance of the luteal phase compared to the follicular phase within healthy, non-smoking, normal-weight, proven normally ovulatory, premenopausal women with normal-length menstrual cycles?

Summary answer: Prospective 1-year data from 53 premenopausal women with two proven normal-length (21-36 days) and normally ovulatory (≥10 days luteal) menstrual cycles upon enrollment showed that, despite 29% of all cycles having incident ovulatory disturbances, within-woman follicular phase length variances were significantly greater than luteal phase length variances.

What is known already: Many studies report menstrual cycle variability, yet few describe variability in follicular and luteal phase lengths. Luteal lengths are assumed 'fixed' at 13-14 days. Most studies have described follicular and luteal phase variability between-women.

Study design, size, duration: This study was a prospective, 1-year, observational cohort study of relative follicular and luteal phase variability both between and within community-dwelling women with two documented normal-length (21-36 days) and normally ovulatory (≥10 days luteal phase) menstrual cycles prior to enrollment. Eighty-one women enrolled in the study and 66 women completed the 1-year study. This study analyzed data from 53 women with complete data for ≥8 cycles (mean 13).

Participants/materials, setting, methods: Participants were healthy, non-smoking, of normal BMI, ages 21-41 with two documented normal-length (21-36 days) and normally ovulatory (≥10 days luteal phase) menstrual cycles prior to enrollment. Participants recorded first morning temperature, exercise durations, and menstrual cycle/life experiences daily in the Menstrual Cycle Diary. We analyzed 694 cycles utilizing a twice-validated least-squares Quantitative Basal Temperature method to determine follicular and luteal phase lengths. Statistical analysis compared relative follicular and luteal phase variance in ovulatory cycles both between-women and within-woman. Normal-length cycles with short luteal phases or anovulation were considered to have subclinical ovulatory disturbances (SOD).

Main results and the role of chance: The 1-year overall 53-woman, 676 ovulatory cycle variances for menstrual cycle, follicular, and luteal phase lengths were 10.3, 11.2, and 4.3 days, respectively. Median variances within-woman for cycle, follicular, and luteal lengths were 3.1, 5.2, and 3.0 days, respectively. Menstrual cycles were largely of normal lengths (98%) with an important prevalence of SOD: 55% of women experienced >1 short luteal phase (<10 days) and 17% experienced at least one anovulatory cycle. Within-woman follicular phase length variances were greater than luteal phase length variances (P < 0.001). However, follicular (P = 0.008) and luteal phase length (P = 0.001) variances, without differences in cycle lengths, were greater in women experiencing any anovulatory cycles (n = 8) than in women with entirely normally ovulatory cycles (n = 6).

Limitations, reasons for caution: Limitations of this study include the relatively small cohort, that most women were White, initially had a normal BMI, and the original cohort required two normal-length and normally ovulatory menstrual cycles before enrollment. Thus, this cohort's data underestimated population menstrual cycle phase variances and the prevalence of SOD.

Wider implications of the findings: Our results reinforce previous findings that the follicular phase is more variable than the luteal phase in premenopausal women with normal-length and ovulatory menstrual cycles. However, our study adds to the growing body of evidence that the luteal phase is not predictably 13-14 days long.

Study funding/competing interest(s): This medical education project of the University of British Columbia was funded by donations to the Centre for Menstrual Cycle and Ovulation Research. The authors do not have any conflicts of interest to disclose.

Trial registration number: N/A.

Keywords: anovulatory cycles; follicular phase length; luteal phase length; menstrual cycle length; normally ovulatory cycles; short luteal phase cycles; variability.

Figure 1.

Flow of participants through the prospective ovulation cohort to study cohort of 53 women who had two normally ovulatory, normal-length menstrual cycles (21–36 days with luteal phase ≥10 days) on enrollment and each contributed ≥8 menstrual cycles of which 694 of 720 cycles had ovulation data.

Figure 2.

Comparison of menstrual cycle lengths in all ovulatory (n = 676) and all anovulatory (n = 18) cycles over 1 year by box and whisker plots in 53 premenopausal women in the Prospective Ovulation Cohort. The line within the box is the median with the lower border being the 25th and the upper the 75th percentile of the data. The extremes of the whiskers show the data’s minimum and maximum values. Outliers are designated by superscript o and an extreme outlier by superscript *. Luteal phase lengths of ≥10 days were normal based on least-squares Quantitative Basal Temperature (LS-QBT©) analysis of a first-morning significant temperature increase that occurred 2.4 days following the serum LH peak (Prior et al., 1990b).

Figure 3.

Luteal phase length frequency distribution in all ovulatory cycles (n = 676) over about 1 year in 53 premenopausal women in the Prospective Ovulation Cohort. Luteal phase lengths of ≥10 days were normal based on least-squares Quantitative Basal Temperature (LS-QBT©) analysis of a first-morning significant temperature increase that occurred 2.4 days following the serum LH peak (Prior et al., 1990b).

Figure 4.

Variances between women of the follicular and luteal phase (FP and LP) lengths in the six women within the Prospective Ovulation Cohort who, over about 1 year, experienced all normally ovulatory menstrual cycles versus the same variances in the eight women (one outlier excluded) who experienced any anovulatory as well as normal (≥10 days) and short luteal phase ovulatory cycles. The line within the box is the median with the lower border being the 25th and the upper the 75th percentile of the data. The extremes of the whiskers show the data’s minimum and maximum values. Luteal phase lengths of ≥10 days were normal based on least-squares Quantitative Basal Temperature (LS-QBT©) analysis of a first-morning significant temperature increase that occurred 2.4 days following the serum LH peak (Prior et al., 1990b).