Influence of race/ethnicity, body mass index, and proximity of menopause on menstrual cycle patterns in the menopausal transition: the Study of Women's Health Across the Nation.

OBJECTIVE: Few studies have evaluated factors that influence menstrual cycle length (MCL) during the menopausal transition (MT), a life stage during which very long cycles become more likely to occur. The objective of this article was to assess how body mass index and race/ethnicity--factors associated with MCL in young women--influence MCL during the MT.
METHODS: Study of Women's Health Across the Nation menstrual calendar substudy data of African-American, white, Chinese, and Japanese women were available for three sites (southeastern Michigan, Los Angeles, and northern California). Self-recorded monthly menstrual calendars with end-of-the-month questions on hormone therapy use and smoking were collected from 1996 to 2006. Height and weight were measured at annual study visits. We used quantile regression to model MCL at the 25th, 50th, 75th, and 90th percentiles with bootstrap sampling to construct 95% CIs. Models evaluated MCL with time indexed to the start of the MT (n = 963) and to the final menstrual period (n = 431).
RESULTS: During the MT, increases in MCL occurred mostly at the right tail of the distribution, reflecting a lengthening of long menstrual cycles, not of the median MCL. After adjustment for smoking, education, physical activity, and time, Chinese and Japanese women had 1 day to 6 days longer MCLs compared with white women. Obese women had 1 day to 5 days longer MCLs compared with nonobese women.
CONCLUSIONS: As occurs in younger women, menstrual characteristics during the MT are influenced by race/ethnicity and obesity. The long menstrual cycles characteristic of the MT are longer in obese women and in Chinese and Japanese women.