I. Dentonkelaar et F. Dewaard, REGULARITY AND LENGTH OF MENSTRUAL CYCLES IN WOMEN AGED 41-46 IN RELATION TO BREAST-CANCER RISK - RESULTS FROM THE DOM-PROJECT, Breast cancer research and treatment, 38(3), 1996, pp. 253-258
The effect of regularity and length of the menstrual cycle on breast c
ancer risk was studied prospectively in 78 cases and 383 age-matched c
ontrols who participated in a breast cancer screening programme, the D
OM-project, in Utrecht, the Netherlands. Before entering the screening
programme when they were aged 41-46, the women kept a menstrual calen
dar during at least three consecutive cycles. Cycles were considered t
o be irregular if any of three cycles was shorter than 21 days or long
er than 35 days and/or if variation between cycle lengths was more tha
n five days. Women with irregular cycles had a significantly reduced r
isk of breast cancer (odds ratio = 0.44; 95% confidence interval 0.22-
0.86) after adjustment for age at menarche, age at first birth, parity
, Quetelet's index and family history of breast cancer. Among regularl
y menstruating women, long cycles (28 days or more) were not significa
ntly associated with increased risk of breast cancer (odds ratio 1.17;
95% confidence interval 0.66-2.09). To the extent that irregular mens
trual cycles reflect anovulatory cycles, our findings support the hypo
thesis that the cumulative number of regular ovulatory cycles increase
s breast cancer risk.