The mean and median cycle lengths in our sample were 28.6 and 28.0 days, respectively (range: 18–55 days). Baseline characteristics of the study sample according to menstrual cycle length are presented in . Shorter cycle length was positively associated with physical activity, current smoking, pack-years of smoking, and having a mother who smoked during pregnancy. Longer cycle length was positively associated with time to cycle regularity and inversely associated with smoking, parity, and caffeine intake. Long duration of menstrual flow (≥7 days) was more prevalent among women with short and long cycles relative to those with average cycle lengths (27–29 days).
Baseline Characteristics of 2,653 Women According to Usual Menstrual Cycle Length, Denmark, 2007–2009
Relative to average cycle lengths (27–29 days), multivariable fecundability ratios for cycle lengths of <25, 25–26, 30–31, 32–33, and ≥34 days were 0.64 (95% confidence interval (CI): 0.49, 0.84), 0.94 (95% CI: 0.77, 1.13), 1.10 (95% CI: 0.97, 1.25), 1.35 (95% CI: 1.06, 1.73), and 1.17 (95% CI: 0.91, 1.49), respectively (). The fecundability ratio comparing ages at menarche of ≥15 with 12–13 years was 0.88 (95% CI: 0.74, 1.05). Compared with cycles that regularized within 2 years after the onset of menses, fecundability ratios for cycles that regularized 2–3 years and ≥4 years after menarche were 0.90 (95% CI: 0.80, 1.02) and 0.89 (95% CI: 0.77, 1.03), respectively. Fecundability ratios were 0.87 (95% CI: 0.72, 1.05) comparing <3 with 3–4 days of menstrual bleeding and 0.70 (95% CI: 0.43, 1.13) comparing very heavy with moderate flow. There was no clear pattern of effect for amount and duration of menstrual flow, when considered jointly, in association with time to pregnancy.
Menstrual Cycle Characteristics and Time to Pregnancy, Denmark, 2007–2009
displays the association of menstrual cycle length with fecundability by using restricted cubic splines (38
). The fecundability ratio increased monotonically with increasing cycle length (P
= 0.31 comparing a linear model with a model containing both linear and spline terms), but the spline model indicated a possible larger influence on fecundability for a 1-unit change in cycle length among shorter cycles compared with longer cycles.
Figure 1. Association between usual menstrual cycle length and fecundability, fitted by restricted cubic splines, Denmark, 2007–2009. The reference level for the fecundability ratio (FR) is a cycle length of 28 days. The curves are adjusted for age, pack-years (more ...)
The effect of short cycle length (<25 days) was relatively uniform across levels of age, smoking history, parity, body mass index, and attempt time at study entry (). Because menstrual cycles may take several months to “normalize” after cessation of hormonal contraceptives and because women who recently discontinued hormonal contraceptives may have difficulties reporting their menstrual characteristics, we also stratified by use of hormonal methods as their last method of contraception (). Results for short cycle length were similar for both groups. When we further stratified these results according to whether the hormonal contraceptive users had waited “a few months” after discontinuation before attempting to conceive, fecundability ratios (<25 vs. 27–29 days) were similar among the 254 women who waited (fecundability ratio = 0.58, 95% CI: 0.21, 1.28) and the 1,335 women who did not wait (fecundability ratio = 0.68, 95% CI: 0.48, 0.98) to conceive after discontinuation of hormonal methods.
Usual Menstrual Cycle Length and Time to Pregnancy, by Selected Factors, Denmark, 2007–2009
The time-varying analysis of cycle length and time to pregnancy (TTP) among the 501 women (18.9%) who provided prospective data on cycle length produced results that were generally consistent with those found in the primary analysis, albeit the effect estimates were less precise: Fecundability ratios for cycle lengths of <25, 25–26, 30–31, 32–33, and ≥34 days, relative to 27–29 days, were 0.39 (95% CI: 0.18, 0.86), 0.90 (95% CI: 0.57, 1.41), 1.41 (95% CI: 1.05, 1.91), 1.33 (95% CI: 0.76, 2.33), and 1.49 (95% CI: 0.89, 2.51), respectively. Finally, results were similar when pregnancy losses were excluded from the outcome definition (data not shown).
It is possible that the results could have differed according to the use of home pregnancy tests, as women using such tests have a higher chance of detecting subclinical losses and reporting shorter TTPs than women having their pregnancies first confirmed by a physician. Data on home pregnancy tests were collected only from women reporting viable pregnancies. When we confined events to the 1,717 women using home pregnancy tests to confirm their pregnancies (96% of viable pregnancies), effect estimates were similar to those found in the primary analyses: Fecundability ratios for cycle lengths of <25, 25–26, 30–31, 32–33, and ≥34 days, relative to 27–29 days, were 0.62 (95% CI: 0.47, 0.83), 0.93 (95% CI: 0.77, 1.14), 1.08 (95% CI: 0.94, 1.23), 1.37 (95% CI: 1.06, 1.77), and 1.16 (95% CI: 0.90, 1.49), respectively.