Table 2 Association between menstrual characteristics and risk of uterine leiomyomata

Menarche age, years ^a

  ≤ 11

0.96 (0.62–1.49)

0.96 (0.62–1.48)

  12–13

Reference

Reference

  14–15

0.75 (0.61–0.92)

0.80 (0.65–0.98)

  ≥ 16

0.65 (0.45–0.94)

0.68 (0.47–0.99)

  P for trend

0.007

0.026

Time to regular cycles, years^b

  ≤ 1

Reference

Reference

  2–4

0.96 (0.75–1.21)

0.92 (0.72–1.17)

  ≥ 5 or always irregular

0.89 (0.71–1.12)

0.87 (0.70–1.10)

  P for trend

0.341

0.253

Menstrual cycle regularity^b

  Cycle regularity

    Very regular

1.03 (0.84–1.27)

1.03 (0.84–1.27)

    Regular

Reference

Reference

    Usually irregular

0.78 (0.58–1.04)

0.77 (0.57–1.03)

    Always irregular

0.79 (0.51–1.23)

0.78 (0.50–1.21)

    P for trend

0.064

0.052

  Cycle regularity ages 18–22 years

    Very regular

1.11 (0.90–1.37)

1.09 (0.88–1.35)

    Regular

Reference

Reference

    Usually irregular

0.78 (0.60–1.03)

0.78 (0.59–1.02)

    Always irregular

0.62 (0.39–0.97)

0.61 (0.39–0.97)

    P for trend

0.002

0.003

Menstrual cycle length^b

  Cycle length, days

    < 26

0.95 (0.69–1.32)

0.96 (0.69–1.32)

    26–31

Reference

Reference

    32–39

0.85 (0.67–1.08)

0.85 (0.67–1.08)

    ≥ 40 or too irregular to estimate

0.41 (0.25–0.68)

0.40 (0.24–0.66)

    P for trend

0.001

< 0.001

  Cycle length ages 18–22 years, days

    < 26

0.92 (0.66–1.29)

0.91 (0.65–1.28)

    26–31

Reference

Reference

    32–39

0.71 (0.55–0.92)

0.73 (0.56–0.94)

    ≥ 40 or too irregular to estimate

0.45 (0.31–0.67)

0.45 (0.31–0.67)

    P for trend

< 0.001

< 0.001