Table 4 Estimates and percentage change (95%CIs) of within-individual standard deviations (SDs) by age, ethnicity, and BMI in 163,275 cycles by 11,040 participants who contributed at least three complete menstrual cycles in the AWHS.

From: Menstrual cycle length variation by demographic characteristics from the Apple Women’s Health Study

 

Estimates (days)

Adjusted percentage difference (95%CIs)

Age (years)

Under 20

5.33 (5.16, 5.51)

45.51 (42.82, 48.20)

20–24

5.07 (4.96, 5.18)

37.33 (34.12, 40.54)

25–29

4.70 (4.62, 4.79)

25.17 (22.24, 28.10)

30–34

4.28 (4.21, 4.34)

12.67 (9.71, 15.63)

35–39

3.79 (3.77, 3.82)

Reference

40–44

3.99 (3.94, 4.04)

5.77 (1.26, 10.28)

45–49

5.42 (5.27, 5.57)

44.54 (40.52, 48.57)

Above 50

11.19 (8.94, 13.45)

200.31 (191.10, 209.52)

Ethnicity

White

4.81 (4.79, 4.83)

Reference

Black

4.67 (4.60, 4.74)

−2.55 (−5.05, −0.04)

Asian

5.04 (4.95, 5.13)

8.91 (3.09, 14.74)

Hispanic

5.09 (5.01, 5.17)

9.96 (3.18, 16.74)

Other

4.63 (4.57, 4.68)

−4.51 (−9.35, 0.34)

More than one ethnicity

4.58 (4.50, 4.66)

−5.39 (−15.31, 4.54)

BMI

Underweight

4.89 (4.74, 5.03)

8.60 (2.45, 14.75)

Healthy

4.57 (4.55, 4.60)

Reference

Overweight

4.82 (4.77, 4.87)

4.30 (1.87, 6.73)

Class 1 obese

5.03 (4.96, 5.10)

12.21 (9.78, 14.63)

Class 2 obese

4.77 (4.69, 4.85)

9.82 (6.21, 13.43)

Class 3 obese

5.43 (5.32, 5.54)

26.90 (23.41, 30.38)

  1. Within-individual cycle length standard deviation estimated from the univariate log-linear model for residual variance in a linear mixed effect model with all covariates in the fixed effect terms and participant-specific random intercepts.
  2. Percentage difference in within-individual cycle length standard deviation estimated from the log-linear model for residual variance exclusively adjusted for age, ethnicity, and BMI. This log-linear model was embedded in a linear mixed effect model with all covariates in the fixed effect terms and participant-specific random intercepts. Missing values in age, ethnicity, and BMI were excluded. Missing values for other covariates (included in the fixed effect terms) were treated with missing indicator.
  3. The other ethnicity category includes American Indian or Alaska Native, Middle Eastern or North African, Native Hawaiian or Pacific Islander, or other unspecified ethnicity.
  4. Underweight was defined as BMI < 18.5 kg/m2. Healthy BMI was defined as 18.5 ≤ BMI < 25 kg/m2. Overweight was defined as 25 ≤ BMI < 30 kg/m2. Class 1 obese was defined as 30 ≤ BMI < 35 kg/m2. Class 2 obese was defined as 35 ≤ BMI < 40 kg/m2. Class 3 obese was defined as BMI ≥ 40 kg/m2.
  5. AWHS Apple Women’s Health Study, BMI body mass index.
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